Now use the teco_spawn_check_wait_status(), which was previously in spawn.c,
so we no longer have to handle the UNIX-specific WEXITSTATUS() macros.

* Simplifies the common task of querying an integer or string from a Q-Register table.
* Undefined Q-Regs are reported as TECO_ERROR_QREGUNDEF, so you can theoretically
  handle this case. In practice however, it requires less boilerplating to
  just call teco_qreg_table_find() manually.
* It also doesn't make sense to use these functions when getting and setting
  a register at the same time as you will want to avoid repeated lookups.

* The main interface is the `FT` command.
  `FT` was an undocumented Video TECO command for etags/ctags lookups.
  I don't want to exactly copy its interface, though.
* `FT` allows looking up symbol names,
  definitions and references.
* ctags will be supported via ctags-lsp.
  LSP support is more powerful though and works without
  regenerating TAGS files all the time.
  The LSP will also allow you to customize auto-completions
  using SciTECO itself (i.e. by writing a language server
  in SciTECO).
* For multiple results, `FT$` can be used to cycle through
  results - this should mimic repeated `S$` or `N$`.
* `:FT...$` does a fuzzy search. IMHO it's not important
  to return a status integer instead. `FT` will only
  really be used in interactive mode.
* Document synchronization is supported via hooks from
  ring.c and via Scintilla notifications.
* Currently, the LSP communication is based on blocking
  GIOChannels. This means that a misbehaving hanging
  server could "lock up" the entire editor (FIXME).
  Only on ncurses you can always kill the subprocess by
  pressing CTRL+C.
  We need helper functions in spawn.c to read and write
  with interruptions.
* The textDocument/didChange notification transmits
  not only all edits, but all files' contents as well
  during initial synchronization.
  Therefore it is optimized to write and JSON-escape
  data without copying them around in memory and without
  destroying the buffer gap.
* Use $SCITECO_LSP to configure the language server.
  You can also use `tee` to capture stdin and stdout.
  Perhaps $SCITECO_LSP should be saved in .teco_session,
  so you can change it between projects?
* $SCITECO_LSP_ROOT is used to point to the project's
  root directory. session.tes will set it up automatically.

It can also be useful for spawning $SCITECO_CLIPBOARD_SET/GET processes (currently via popen())
and for launching language servers.

Previously almost all glyph-to-byte offset conversions consulted
Scintilla's line index and counted characters on the resulting line.
For instance a simple expression like `.+1J` would scan the same line
twice completely, which would be very slow on pathologically long lines.
Even insertions did that due to having to update the ^Y ranges.
If you repeat such an operation over all characters as in `<.+1:J;>`
you would have complexity O(n^2) for n = line length.
Only commands with an explicit relative nature like `C` and `A` would
use teco_view_glyph2bytes_relative() which scans beginning at dot
as long as the relative movement is less than 1024 glyphs.

Wit the new heuristics almost all glyph-to-byte and byte-to-glyph
conversions can make use of that optimization.
This requires that dot must at all times be known in glyphs as well -
the byte position is managed by Scintilla (SCI_GETCURRENTPOS).
We therefore introduced teco_current_doc_set_dot() and
teco_current_doc_get_dot() to update dot in the current buffer or
Q-Register -- it cannot be stored along with the view since
Q-Registers share a single view.
A number of auxiliary functions have been introduced for
converting relative to a known (glyphs,bytes) offset pair
and for converting absolute and relative positions with regard
to the current doc and SCI_GETCURRENTPOS position.
Of course this is error-prone since the glyph and dot positions
are interdependant - they must always be kept in sync.

With these new optimizations even pathologically long lines can
(usually) be managed even in UTF-8 documents.
It does not address slow-downs in Scintilla's line layout, yet.
grosciteco.tes for instance runs twice as fast now.

* terex disables assertions by default unless you add `-DREG_DEBUG`.
  Since we heavily modified the original by Henry Spencer it makes sense
  to enable assertions.
* dlmalloc will still be built without assertions even if --enable-debug
  is given since that has a significant speed impact and I consider
  dlmalloc to be rock solid. It would need `-DDEBUG=1` to enable assertions
  (among other things).
  We only disable additional checks in dlmalloc if --disable-debug.

* GSpawn ends up calling posix_spawnp() which passes down a small
  4kb stack to the child process until it exec()s.
  This stack could be overflowed easily on code paths where the
  path is not already absolute and when many shared libraries
  are involved.
* The crashes could therefore only be observed on Gtk builds and
  in UNIX shell emulation mode (0,128ED). Sample test case:
  gsciteco -e '0,128ED @EC"ls"'
  Theoretically a relative $SHELL variable could have also triggered
  it.
* I assume that the bug will be fixed in libc at least by the time of
  FreeBSD 16.
* As a workaround we resolve relative program paths before passing
  them to g_spawn_async_with_pipes().

* SIGTERM used to insert the ^KCLOSE key macro.
  However with the default ^KCLOSE macro, which inserts `EX`,
  this may fail to terminate the editor if buffers are modified.
  If the process is consequently killed by a non-ignorable signal,
  we may still loose data.
* SIGTERM is used to gracefully shut down, so we now always terminate.
  Since we have recovery files, they are now dumped before terminating.
  This makes sure that recovery files are more up-to-date during
  unexpected but gracefull terminations.
* The same functionality is planned on Curses, but requires more fundamental
  changes (TODO).