Provide sigaction() for Windows.

Commit 9abb2bfc left behind code to block signals inside signal
handlers on Windows, because our signal porting layer didn't have
sigaction().  Provide a minimal implementation that is capable of
blocking signals, to get rid of platform differences.  See also related
commit c94ae9d8.

Discussion: https://postgr.es/m/CA%2BhUKGKKKfcgx6jzok9AYenp2TNti_tfs8FMoJpL8%2B0Gsy%3D%3D_A%40mail.gmail.com

src/port/pqsignal.c

@@ -29,7 +29,9 @@
 
 #include <signal.h>
 
-#if !defined(WIN32) || defined(FRONTEND)
+#ifndef FRONTEND
+#include "libpq/pqsignal.h"
+#endif
 
 /*
  * Set up a signal handler, with SA_RESTART, for signal "signo"
@@ -39,7 +41,7 @@
 pqsigfunc
 pqsignal(int signo, pqsigfunc func)
 {
-#ifndef WIN32
+#if !(defined(WIN32) && defined(FRONTEND))
 	struct sigaction act,
 				oact;
 
@@ -53,9 +55,8 @@ pqsignal(int signo, pqsigfunc func)
 	if (sigaction(signo, &act, &oact) < 0)
 		return SIG_ERR;
 	return oact.sa_handler;
-#else							/* WIN32 */
+#else
+	/* Forward to Windows native signal system. */
 	return signal(signo, func);
 #endif
 }
-
-#endif							/* !defined(WIN32) || defined(FRONTEND) */