patch 9.1.0264: libgpm may delete some signal handlers

patch 9.1.0264: libgpm may delete some signal handlers

Problem:  libgpm may delete some signal handlers
Solution: restore these signal handlers after calling gpm
          (Julio B)

When 'mouse' is set, vim is trying to detect mouse support on startup.
Eventually, vim resorts to using libgpm as the final method of
mouse detection. This library may delete some signals handlers that were
initially set up by vim.

This is how:
- mch_setmouse() calls gpm_open()
- Gpm_Open is executed, which returns early on line 210 [1]
- Keep in mind that lines 353-373 [2] are skipped, so
  gpm_saved_suspend_hook and gpm_saved_winch_hook are empty
- Finally, Gpm_Close is called, which will reset [3] SIGWINCH and
  SIGTSTP to an empty sigaction.

[1] https://github.com/telmich/gpm/blob/e82d1a653ca94aa4ed12441424da6ce780b1e530/src/lib/liblow.c#L210
[2] https://github.com/telmich/gpm/blob/e82d1a653ca94aa4ed12441424da6ce780b1e530/src/lib/liblow.c#L353-L373
[3] https://github.com/telmich/gpm/blob/e82d1a653ca94aa4ed12441424da6ce780b1e530/src/lib/liblow.c#L419-L424

fixes: #12154
closes: #14401

Signed-off-by: Julio B <julio.bacel@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>