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);