compat/mingw.c: expand support for sigaction

Due to portability issues across UNIX versions sigaction(2) should be used
instead of signal(2).

From the signal(2) man page:

  The behavior of signal() varies across UNIX versions, and has also var‐
  ied historically across different versions of Linux.   Avoid  its  use:
  use sigaction(2) instead.

Unfortunately MinGW under Windows has limited support for signal and no
support for sigaction.  And this prevents sigaction from being used across
the entire Git project.

In compat/mingw.c there is a faux sigaction function but it only supports
SIGALARM.  Hence the need for continuing to use signal() in other cases.

This patch expands the faux sigaction in compat/mingw.c so that it calls
signal in cases other than SIGALRM.  Now sigaction can be used across the
entire Git project and MinGW will still work with signal.