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>

compat/mingw-posix.h

@@ -96,6 +96,7 @@ struct sigaction {
 	unsigned sa_flags;
 };
 #define SA_RESTART 0
+#define SA_NOCLDSTOP 1
 
 struct itimerval {
 	struct timeval it_value, it_interval;

compat/mingw.c

@@ -2561,7 +2561,9 @@ int setitimer(int type UNUSED, struct itimerval *in, struct itimerval *out)
 
 int sigaction(int sig, struct sigaction *in, struct sigaction *out)
 {
-	if (sig != SIGALRM)
+	if (sig == SIGCHLD)
+		return -1;
+	else if (sig != SIGALRM)
 		return errno = EINVAL,
 			error("sigaction only implemented for SIGALRM");
 	if (out)