Use SIGURG rather than SIGUSR1 for latches.

Traditionally, SIGUSR1 has been overloaded for ad-hoc signals,
procsignal.c signals and latch.c wakeups.  Move that last use over to a
new dedicated signal.  SIGURG is normally used to report out-of-band
socket data, but PostgreSQL doesn't use that facility.

The signal handler is now installed in all postmaster children by
InitializeLatchSupport().  Those wishing to disconnect from it should
call ShutdownLatchSupport().

Future patches will use this separation of signals to avoid the need for
a signal handler on some operating systems.

Discussion: https://postgr.es/m/CA+hUKGJjxPDpzBE0a3hyUywBvaZuC89yx3jK9RFZgfv_KHU7gg@mail.gmail.com

src/backend/postmaster/postmaster.c

@@ -656,6 +656,10 @@ PostmasterMain(int argc, char *argv[])
 	pqsignal_pm(SIGUSR2, dummy_handler);	/* unused, reserve for children */
 	pqsignal_pm(SIGCHLD, reaper);	/* handle child termination */
 
+#ifdef SIGURG
+	pqsignal_pm(SIGURG, SIG_IGN);	/* ignored */
+#endif
+
 	/*
 	 * No other place in Postgres should touch SIGTTIN/SIGTTOU handling.  We
 	 * ignore those signals in a postmaster environment, so that there is no