Stop bgworkers during fast shutdown with postmaster in startup phase

When a postmaster gets into its phase PM_STARTUP, it would start
background workers using BgWorkerStart_PostmasterStart mode immediately,
which would cause problems for a fast shutdown as the postmaster forgets
to send SIGTERM to already-started background workers.  With smart and
immediate shutdowns, this correctly happened, and fast shutdown is the
only mode missing the shot.

Author: Alexander Kukushkin
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/CAFh8B=mvnD8+DZUfzpi50DoaDfZRDfd7S=gwj5vU9GYn8UvHkA@mail.gmail.com
Backpatch-through: 9.5

src/backend/postmaster/postmaster.c

@@ -2685,7 +2685,7 @@ pmdie(SIGNAL_ARGS)
 				signal_child(BgWriterPID, SIGTERM);
 			if (WalReceiverPID != 0)
 				signal_child(WalReceiverPID, SIGTERM);
-			if (pmState == PM_RECOVERY)
+			if (pmState == PM_STARTUP || pmState == PM_RECOVERY)
 			{
 				SignalSomeChildren(SIGTERM, BACKEND_TYPE_BGWORKER);