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.

These can fail, so abort() just like glib does internally in case of OOM.
A special case is the pad allocation for autocompletion popups, which
can have excessively many lines. This is handled gracefully now
(the popup simply won't be displayed).

But I doubt that get_string() could in practice fail on the plain
environment variable Q-Regs. So this was never triggered.

* The terminal's default foreground and background colors
  are now used by default (`sciteco --no-profile`), so SciTECO
  integrates naturally into all terminal color schemes, even
  dark-on-bright ones.
* The default Scintilla colors use only 0x000000 (COLOR_BLACK) and 0xC0C0C0 (COLOR_WHITE)
  now.
* You can use `7EJ` to configure the default colors in color
  schemes or your profile.
  All existing color schemes had to disable default colors
  (`-1,-1,7EJ`) since they wouldn't look well otherwise.
* You may add `-1,7EJ` to ~/.teco_ini when using a terminal emulator
  with a washed-out palettized COLOR_BLACK.
  We cannot detect the terminal's default colors automatically.
* Scinterm updated to v6.0.
  We require a not-yet-upstreamed patch:
  https://github.com/orbitalquark/scinterm/pull/40
* In fact, we might decide not to support default colors at all in Scinterm,
  so this feature should be considered experimental.

* Instead of supporting only 16 predefined RGB placeholder
  values in Scintilla messages and styles, you can now use
  arbitrary RGB values and colors are allocated via the terminal
  on the fly.
  You no longer need to call 3EJ to change the default color
  palette.
* The placeholder RGB values are still available.
  Since you will usually want exact RGB values when using
  anything outside of the range of 16 default colors
  and the RGB placeholders will not always exactly correspond
  to their RGB value, you can now call `0,3EJ` to ignore
  the default palette and allocate all colors dynamically.
* Allows for more than 16 colors on the screen simultaneously.
  Also simplifies the solarized.tes color scheme.
  Since both Scinterm and SciTECO try not to touch the 16
  default colors, you also no longer have to deal with
  restoring the palette after program termination
  (which was never reliable anyway).
* Color schemes with non-default colors (solarized.tes)
  may now be broken on TERM=linux-16color (Linux VT)
  since Scinterm will get only 8 colors, but solarized.tes
  needs 16.

* Scinterm was simply rendering them as black, thus effectively
  breaking the Linux and FreeBSD vts with terminal.tes.
* I was considering to render light black as white on 8-color terminals,
  so it's always readable.
  However, if you add in A_BOLD there is a good chance that the
  color will end up grey - at least it does in the virtual terminals (consoles).
* There is no need to use bright colors in the Scintilla view defaults.
  E.g. 0xFFFFF is "light white".
  However on 8-color terminals this will be rendered like white anyway.
  The new defaults are closer to what terminal.tes does.

* 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).

* There was a logic error in teco_interface_getch() that caused Curses function key
  codes to be returned directly. These codes however are useless to macro authors and
  can be confused with codepoints. You cannot report function keys in the same "namespace"
  along with Unicode codepoints.
  They are now filtered out.
* Also make sure that Backspace and Return are reported as 8 and 10 respectively
  in all Curses variants.
  All control codes reported by Curses are passed down unmodified - in contrast to
  the command-line input handling. I.e. 13 is not normalized to 10.
* PDCursesMod/WinGUI may return bogus key presses, that also have to be filtered out
  as we already did in the main input handling.
  A function teco_interface_check_key() has been introduced.
* NOTE: teco_interface_blocking_getch() already makes sure that recovery files are dumped
  even when blocking in `^T`.