compat/mingw: allow sigaction(SIGCHLD)

compat/mingw: allow sigaction(SIGCHLD)

A future change will start using sigaction to setup a SIGCHLD signal
handler.

The current code uses signal(), which returns SIG_ERR (but doesn't
seem to set errno) so instruct sigaction() to do the same.

A new SA flag will be needed, so copy the one from Cygwin; note that
the sigaction() implementation that is provided won't use it, so
its value is otherwise irrelevant.

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>