This is a total conversion of SciTECO to plain C (GNU C11).
The chance was taken to improve a lot of internal datastructures,
fix fundamental bugs and lay the foundations of future features.
The GTK user interface is now in an useable state!
All changes have been squashed together.

The language itself has almost not changed at all, except for:

* Detection of string terminators (usually Escape) now takes
  the string building characters into account.
  A string is only terminated outside of string building characters.
  In other words, you can now for instance write
  I^EQ[Hello$world]$
  This removes one of the last bits of shellisms which is out of
  place in SciTECO where no tokenization/lexing is performed.
  Consequently, the current termination character can also be
  escaped using ^Q/^R.
  This is used by auto completions to make sure that strings
  are inserted verbatim and without unwanted sideeffects.
* All strings can now safely contain null-characters
  (see also: 8-bit cleanliness).
  The null-character itself (^@) is not (yet) a valid SciTECO
  command, though.

An incomplete list of changes:

* We got rid of the BSD headers for RB trees and lists/queues.
  The problem with them was that they used a form of metaprogramming
  only to gain a bit of type safety. It also resulted in less
  readble code. This was a C++ desease.
  The new code avoids metaprogramming only to gain type safety.
  The BSD tree.h has been replaced by rb3ptr by Jens Stimpfle
  (https://github.com/jstimpfle/rb3ptr).
  This implementation is also more memory efficient than BSD's.
  The BSD list.h and queue.h has been replaced with a custom
  src/list.h.
* Fixed crashes, performance issues and compatibility issues with
  the Gtk 3 User Interface.
  It is now more or less ready for general use.
  The GDK lock is no longer used to avoid using deprecated functions.
  On the downside, the new implementation (driving the Gtk event loop
  stepwise) is even slower than the old one.
  A few glitches remain (see TODO), but it is hoped that they will
  be resolved by the Scintilla update which will be performed soon.
* A lot of program units have been split up, so they are shorter
  and easier to maintain: core-commands.c, qreg-commands.c,
  goto-commands.c, file-utils.h.
* Parser states are simply structs of callbacks now.
  They still use a kind of polymorphy using a preprocessor trick.
  TECO_DEFINE_STATE() takes an initializer list that will be
  merged with the default list of field initializers.
  To "subclass" states, you can simply define new macros that add
  initializers to existing macros.
* Parsers no longer have a "transitions" table but the input_cb()
  may use switch-case statements.
  There are also teco_machine_main_transition_t now which can
  be used to implement simple transitions. Additionally, you
  can specify functions to execute during transitions.
  This largely avoids long switch-case-statements.
* Parsers are embeddable/reusable now, at least in parse-only mode.
  This does not currently bring any advantages but may later
  be used to write a Scintilla lexer for TECO syntax highlighting.
  Once parsers are fully embeddable, it will also be possible
  to run TECO macros in a kind of coroutine which would allow
  them to process string arguments in real time.
* undo.[ch] still uses metaprogramming extensively but via
  the C preprocessor of course. On the downside, most undo
  token generators must be initiated explicitly (theoretically
  we could have used embedded functions / trampolines to
  instantiate automatically but this has turned out to be
  dangereous).
  There is a TECO_DEFINE_UNDO_CALL() to generate closures for
  arbitrary functions now (ie. to call an arbitrary function
  at undo-time). This simplified a lot of code and is much
  shorter than manually pushing undo tokens in many cases.
* Instead of the ridiculous C++ Curiously Recurring Template
  Pattern to achieve static polymorphy for user interface
  implementations, we now simply declare all functions to
  implement in interface.h and link in the implementations.
  This is possible since we no longer hace to define
  interface subclasses (all state is static variables in
  the interface's *.c files).
* Headers are now significantly shorter than in C++ since
  we can often hide more of our "class" implementations.
* Memory counting is based on dlmalloc for most platforms now.
  Unfortunately, there is no malloc implementation that
  provides an efficient constant-time memory counter that
  is guaranteed to decrease when freeing memory.
  But since we use a defined malloc implementation now,
  malloc_usable_size() can be used safely for tracking memory use.
  malloc() replacement is very tricky on Windows, so we
  use a poll thread on Windows. This can also be enabled
  on other supported platforms using --disable-malloc-replacement.
  All in all, I'm still not pleased with the state of memory
  limiting. It is a mess.
* Error handling uses GError now. This has the advantage that
  the GError codes can be reused once we support error catching
  in the SciTECO language.
* Added a few more test suite cases.
* Haiku is no longer supported as builds are instable and
  I did not manage to debug them - quite possibly Haiku bugs
  were responsible.
* Glib v2.44 or later are now required.
  The GTK UI requires Gtk+ v3.12 or later now.
  The GtkFlowBox fallback and sciteco-wrapper workaround are
  no longer required.
* We now extensively use the GCC/Clang-specific g_auto
  feature (automatic deallocations when leaving the current
  code block).
* Updated copyright to 2021.
  SciTECO has been in continuous development, even though there
  have been no commits since 2018.
* Since these changes are so significant, the target release has
  been set to v2.0.
  It is planned that beginning with v3.0, the language will be
  kept stable.

 * it turns out that option-like arguments could not be reliably passed to
   SciTECO scripts for two reasons:
   a) "--" arguments are not removed from argv by GOption if it detects
      and following option-like argument.
      "--" would thus be passed as a script argument which will disable
      option parsing in scripts that interpret "--".
   b) A script run via the Hash-Bang line "#!...sciteco -m" would
      require an explicit "--" to turn of GOption parsing.
      However it is __impossible__ to insert after the script file name
      on UNIX.
 * Therefore, SciTECO now removes leading "--" arguments left over by GOption.
 * If possible (Glib >= 2.44), option parsing is performed in strict POSIX
   mode which inhibits parsing after the first non-option argument.
   This reduces the number of cases where an explicit "--" is required.
 * --mung no longer takes an argument. Instead, the first non-option argument
   is expected to be the script file name.
   This looks weird at first but is more consistent with how other interpeters
   work. Once we revise argument passing to scripts, the script name can also
   be passed to the script which is more consistent with it being the first
   non-option argument.
   Also, with strict POSIX parsing, this fixed Hash-Bang lines since
   the script file name constructed by the kernel will automatically switch
   off option parsing, passing all option-like script arguments uninterpreted
   to the script.
 * Since we're supporting Glib < 2.44, the Hash-Bang lines are still broken
   for certain builds.
   Therefore, a wrapper script is installed to libexecdir (it never has to be
   executed by users and Hash-Bang lines need absolute paths anyway) which
   transparently inserts "--" into the SciTECO command line and should be used
   as the interpreter in portable SciTECO scripts.
   The wrapper script is generated and points to the exact SciTECO binary
   installed. This is important when doing parallel installs of Curses and Gtk
   binaries since each one will get its own working wrapper script.
   The wrapper-script workaround can be removed once we depend on Glib >= 2.44
   (some day...).
 * The default /usr/bin/env Hash-Bang lines are no longer used in the
   scripts since they are broken anyway (UNIX incl. Linux cannot pass
   multiple arguments to the interpreter!).
   Scripts that get installed will get a fixed-up Hash-Bang line referring
   to the installed SciTECO binary anyway.
 * Interface::main() has been renamed to Interface::init() and is optional
   now. The Interface::main() method was introduced because of the misconception
   that interfaces will find their options in the argv array and have to do
   their own parsing.
   This is wrong, since their option group already cares about parsing.
   Therefore, gtk_init() does not have to called explicitly, too.

 * mapped to different registers beginning with "~"
 * on supported platforms accessing the clipboard is as easy as
   X~ or G~.
   Naturally this also allows clipboards to be pasted in
   string arguments/insertions (^EQ~).
 * Currently, Gtk+, PDCurses and ncurses/XTerm are supported.
   For XTerm clipboard support, users must set 0,256ED to enable
   it since we cannot check for XTerm window ops programmatically
   (at least without libX11).
 * When clipboard regs exist, the clipboard can also be deemed functional.
   This allows macros to fall back to xclip(1) if necessary.
 * EOL handling has been moved into a new file eol.c and eol.h.
   EOL translation no longer depends on GIOChannels but can be
   memory-backed as well.

 * this has been prepared a long time ago
 * the popup widget does not in any way depend on the InterfaceCurses
   class and could be used elsewhere.
 * common and generic Curses drawing functions required by both the
   Curses UI and the CursesInfoPopup widget have been factored out
   into curses-utils.cpp (namespace Curses)
 * this improved the UI-logic separation and helped in making
   interface-curses.cpp smaller

 * use libtool convenience libraries as much as possible
   (for all static libraries except Scintilla)
 * improves separation of language and user interface implementations
   (e.g. the Gtk widgets are not interesting for the rest of SciTECO)
 * the Curses popup widget can now be factored out of interface-curses.cpp
 * some common CPPFLAGS are now defined by ./configure via AM_CPPFLAGS,
   so they don't have to be repeated in each submodule.
 * fixed building the Curses UI: GTK_FLOW_BOX_FALLBACK conditional
   must always be defined.