Improve client error messages for immediate-stop situations.

Up to now, if the DBA issued "pg_ctl stop -m immediate", the message
sent to clients was the same as for a crash-and-restart situation.
This is confusing, not least because the message claims that the
database will soon be up again, something we have no business
predicting.

Improve things so that we can generate distinct messages for the two
cases (and also recognize an ad-hoc SIGQUIT, should somebody try that).
To do that, add a field to pmsignal.c's shared memory data structure
that the postmaster sets just before broadcasting SIGQUIT to its
children.  No interlocking seems to be necessary; the intervening
signal-sending and signal-receipt should sufficiently serialize accesses
to the field.  Hence, this isn't any riskier than the existing usages
of pmsignal.c.

We might in future extend this idea to improve other
postmaster-to-children signal scenarios, although none of them
currently seem to be as badly overloaded as SIGQUIT.

Discussion: https://postgr.es/m/559291.1608587013@sss.pgh.pa.us

src/backend/postmaster/postmaster.c

@@ -218,6 +218,7 @@ int			ReservedBackends;
 /* The socket(s) we're listening to. */
 #define MAXLISTEN	64
 static pgsocket ListenSocket[MAXLISTEN];
+
 /*
  * These globals control the behavior of the postmaster in case some
  * backend dumps core.  Normally, it kills all peers of the dead backend
@@ -2887,6 +2888,8 @@ pmdie(SIGNAL_ARGS)
 			sd_notify(0, "STOPPING=1");
 #endif
 
+			/* tell children to shut down ASAP */
+			SetQuitSignalReason(PMQUIT_FOR_STOP);
 			TerminateChildren(SIGQUIT);
 			pmState = PM_WAIT_BACKENDS;
 
@@ -3464,6 +3467,7 @@ HandleChildCrash(int pid, int exitstatus, const char *procname)
 		LogChildExit(LOG, procname, pid, exitstatus);
 		ereport(LOG,
 				(errmsg("terminating any other active server processes")));
+		SetQuitSignalReason(PMQUIT_FOR_CRASH);
 	}
 
 	/* Process background workers. */