Ignore interrupts during quickdie().

Once the administrator has called for an immediate shutdown or a backend
crash has triggered a reinitialization, no mere SIGINT or SIGTERM should
change that course.  Such derailment remains possible when the signal
arrives before quickdie() blocks signals.  That being a narrow race
affecting most PostgreSQL signal handlers in some way, leave it for
another patch.  Back-patch this to all supported versions.

src/backend/tcop/postgres.c

@@ -2488,6 +2488,13 @@ quickdie(SIGNAL_ARGS)
 {
 	PG_SETMASK(&BlockSig);
 
+	/*
+	 * Prevent interrupts while exiting; though we just blocked signals that
+	 * would queue new interrupts, one may have been pending.  We don't want a
+	 * quickdie() downgraded to a mere query cancel.
+	 */
+	HOLD_INTERRUPTS();
+
 	/*
 	 * Ideally this should be ereport(FATAL), but then we'd not get control
 	 * back...