Use WaitEventSet API for postmaster's event loop.

Switch to a design similar to regular backends, instead of the previous
arrangement where signal handlers did non-trivial state management and
called fork().  The main changes are:

* The postmaster now has its own local latch to wait on.  (For now, we
  don't want other backends setting its latch directly, but that could
  probably be made to work with more research on robustness.)

* The existing signal handlers are cut in two: a handle_pm_XXX() part
  that just sets pending_pm_XXX flags and the latch, and a
  process_pm_XXX() part that runs later when the latch is seen.

* Signal handlers are now installed with the regular pqsignal()
  function rather than the special pqsignal_pm() function; historical
  portability concerns about the effect of SA_RESTART on select() are no
  longer relevant, and we don't need to block signals anymore.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKG%2BZ-HpOj1JsO9eWUP%2Bar7npSVinsC_npxSy%2BjdOMsx%3DGg%40mail.gmail.com

src/backend/libpq/pqsignal.c

@@ -97,43 +97,3 @@ pqinitmask(void)
 	sigdelset(&StartupBlockSig, SIGALRM);
 #endif
 }
-
-/*
- * Set up a postmaster signal handler for signal "signo"
- *
- * Returns the previous handler.
- *
- * This is used only in the postmaster, which has its own odd approach to
- * signal handling.  For signals with handlers, we block all signals for the
- * duration of signal handler execution.  We also do not set the SA_RESTART
- * flag; this should be safe given the tiny range of code in which the
- * postmaster ever unblocks signals.
- *
- * pqinitmask() must have been invoked previously.
- */
-pqsigfunc
-pqsignal_pm(int signo, pqsigfunc func)
-{
-	struct sigaction act,
-				oact;
-
-	act.sa_handler = func;
-	if (func == SIG_IGN || func == SIG_DFL)
-	{
-		/* in these cases, act the same as pqsignal() */
-		sigemptyset(&act.sa_mask);
-		act.sa_flags = SA_RESTART;
-	}
-	else
-	{
-		act.sa_mask = BlockSig;
-		act.sa_flags = 0;
-	}
-#ifdef SA_NOCLDSTOP
-	if (signo == SIGCHLD)
-		act.sa_flags |= SA_NOCLDSTOP;
-#endif
-	if (sigaction(signo, &act, &oact) < 0)
-		return SIG_ERR;
-	return oact.sa_handler;
-}