Fix some more bugs in signal handlers and process shutdown logic.

WalSndKill was doing things exactly backwards: it should first clear
MyWalSnd (to stop signal handlers from touching MyWalSnd->latch),
then disown the latch, and only then mark the WalSnd struct unused by
clearing its pid field.

Also, WalRcvSigUsr1Handler and worker_spi_sighup failed to preserve
errno, which is surely a requirement for any signal handler.

Per discussion of recent buildfarm failures.  Back-patch as far
as the relevant code exists.

src/backend/replication/walreceiver.c

@@ -740,7 +740,11 @@ WalRcvSigHupHandler(SIGNAL_ARGS)
 static void
 WalRcvSigUsr1Handler(SIGNAL_ARGS)
 {
+	int			save_errno = errno;
+
 	latch_sigusr1_handler();
+
+	errno = save_errno;
 }
 
 /* SIGTERM: set flag for main loop, or shutdown immediately if safe */