Centralize setup of SIGQUIT handling for postmaster child processes.

We decided that the policy established in commit 7634bd4f6 for
the bgwriter, checkpointer, walwriter, and walreceiver processes,
namely that they should accept SIGQUIT at all times, really ought
to apply uniformly to all postmaster children.  Therefore, get
rid of the duplicative and inconsistent per-process code for
establishing that signal handler and removing SIGQUIT from BlockSig.
Instead, make InitPostmasterChild do it.

The handler set up by InitPostmasterChild is SignalHandlerForCrashExit,
which just summarily does _exit(2).  In interactive backends, we
almost immediately replace that with quickdie, since we would prefer
to try to tell the client that we're dying.  However, this patch is
changing the behavior of autovacuum (both launcher and workers), as
well as walsenders.  Those processes formerly also used quickdie,
but AFAICS that was just mindless copy-and-paste: they don't have
any interactive client that's likely to benefit from being told this.

The stats collector continues to be an outlier, in that it thinks
SIGQUIT means normal exit.  That should probably be changed for
consistency, but there's another patch set where that's being
dealt with, so I didn't do so here.

Discussion: https://postgr.es/m/644875.1599933441@sss.pgh.pa.us

src/backend/tcop/postgres.c

@@ -3820,7 +3820,8 @@ PostgresMain(int argc, char *argv[],
 	}
 
 	/*
-	 * Set up signal handlers and masks.
+	 * Set up signal handlers.  (InitPostmasterChild or InitStandaloneProcess
+	 * has already set up BlockSig and made that the active signal mask.)
 	 *
 	 * Note that postmaster blocked all signals before forking child process,
 	 * so there is no race condition whereby we might receive a signal before
@@ -3842,6 +3843,9 @@ PostgresMain(int argc, char *argv[],
 		pqsignal(SIGTERM, die); /* cancel current query and exit */
 
 		/*
+		 * In a postmaster child backend, replace SignalHandlerForCrashExit
+		 * with quickdie, so we can tell the client we're dying.
+		 *
 		 * In a standalone backend, SIGQUIT can be generated from the keyboard
 		 * easily, while SIGTERM cannot, so we make both signals do die()
 		 * rather than quickdie().
@@ -3871,16 +3875,6 @@ PostgresMain(int argc, char *argv[],
 									 * platforms */
 	}
 
-	pqinitmask();
-
-	if (IsUnderPostmaster)
-	{
-		/* We allow SIGQUIT (quickdie) at all times */
-		sigdelset(&BlockSig, SIGQUIT);
-	}
-
-	PG_SETMASK(&BlockSig);		/* block everything except SIGQUIT */
-
 	if (!IsUnderPostmaster)
 	{
 		/*