Make archiver's SIGQUIT handler exit via _exit().

Commit 8e19a8264 changed the SIGQUIT handlers of almost all server
processes not to run atexit callbacks.  The archiver process was
skipped, perhaps because it's not connected to shared memory; but
it's just as true here that running atexit callbacks in a signal
handler is unsafe.  So let's make it work like the rest.

In HEAD and v13, we can use the common SignalHandlerForCrashExit
handler.  Before that, just tweak pgarch_exit to use _exit(2)
explicitly.

Like the previous commit, back-patch to all supported branches.

Kyotaro Horiguchi, back-patching by me

Discussion: https://postgr.es/m/1850884.1599601164@sss.pgh.pa.us

src/backend/postmaster/pgarch.c

@@ -255,8 +255,16 @@ PgArchiverMain(int argc, char *argv[])
 static void
 pgarch_exit(SIGNAL_ARGS)
 {
-	/* SIGQUIT means curl up and die ... */
-	exit(1);
+	/*
+	 * We DO NOT want to run proc_exit() or atexit() callbacks; they wouldn't
+	 * be safe to run from a signal handler.  Just nail the windows shut and
+	 * get out of town.
+	 *
+	 * For consistency with other postmaster children, we do _exit(2) not
+	 * _exit(1).  The postmaster currently will treat these exit codes alike,
+	 * but it seems better to report that we died in an unexpected way.
+	 */
+	_exit(2);
 }
 
 /* SIGHUP signal handler for archiver process */