Use signalfd(2) for epoll latches.

Cut down on system calls and other overheads by reading from a signalfd
instead of using a signal handler and self-pipe.  Affects Linux sytems,
and possibly others including illumos that implement the Linux epoll and
signalfd interfaces.
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGJjxPDpzBE0a3hyUywBvaZuC89yx3jK9RFZgfv_KHU7gg@mail.gmail.com

src/backend/libpq/pqsignal.c

@@ -35,13 +35,15 @@ sigset_t	UnBlockSig,
  * collection; it's essentially BlockSig minus SIGTERM, SIGQUIT, SIGALRM.
  *
  * UnBlockSig is the set of signals to block when we don't want to block
- * signals (is this ever nonzero??)
+ * signals.
  */
 void
 pqinitmask(void)
 {
 	sigemptyset(&UnBlockSig);
 
+	/* Note: InitializeLatchSupport() modifies UnBlockSig. */
+
 	/* First set all signals, then clear some. */
 	sigfillset(&BlockSig);
 	sigfillset(&StartupBlockSig);

src/backend/utils/init/miscinit.c

@@ -118,6 +118,11 @@ InitPostmasterChild(void)
 	/* We don't want the postmaster's proc_exit() handlers */
 	on_exit_reset();
 
+	/* In EXEC_BACKEND case we will not have inherited BlockSig etc values */
+#ifdef EXEC_BACKEND
+	pqinitmask();
+#endif
+
 	/* Initialize process-local latch support */
 	InitializeLatchSupport();
 	MyLatch = &LocalLatchData;
@@ -135,11 +140,6 @@ InitPostmasterChild(void)
 		elog(FATAL, "setsid() failed: %m");
 #endif
 
-	/* In EXEC_BACKEND case we will not have inherited BlockSig etc values */
-#ifdef EXEC_BACKEND
-	pqinitmask();
-#endif
-
 	/*
 	 * Every postmaster child process is expected to respond promptly to
 	 * SIGQUIT at all times.  Therefore we centrally remove SIGQUIT from