Fix postmaster to attempt restart after a hot-standby crash.

The postmaster was coded to treat any unexpected exit of the startup
process (i.e., the WAL replay process) as a catastrophic crash, and not try
to restart it. This was OK so long as the startup process could not have
any sibling postmaster children.  However, if a hot-standby backend
crashes, we SIGQUIT the startup process along with everything else, and the
resulting exit is hardly "unexpected".  Treating it as such meant we failed
to restart a standby server after any child crash at all, not only a crash
of the WAL replay process as intended.  Adjust that.  Back-patch to 9.0
where hot standby was introduced.

src/backend/postmaster/postmaster.c

@@ -2391,13 +2391,18 @@ reaper(SIGNAL_ARGS)
 			}
 
 			/*
-			 * Any unexpected exit (including FATAL exit) of the startup
-			 * process is treated as a crash, except that we don't want to
-			 * reinitialize.
+			 * After PM_STARTUP, any unexpected exit (including FATAL exit) of
+			 * the startup process is catastrophic, so kill other children,
+			 * and set RecoveryError so we don't try to reinitialize after
+			 * they're gone.  Exception: if FatalError is already set, that
+			 * implies we previously sent the startup process a SIGQUIT, so
+			 * that's probably the reason it died, and we do want to try to
+			 * restart in that case.
 			 */
 			if (!EXIT_STATUS_0(exitstatus))
 			{
-				RecoveryError = true;
+				if (!FatalError)
+					RecoveryError = true;
 				HandleChildCrash(pid, exitstatus,
 								 _("startup process"));
 				continue;