directory: update do its work inside a thread

A lot of the preparation was needed (and done in previous
months) in making update thread-safe, but here it is.

This was the first thing I made work inside a thread when I
started mpd-uclinux many years ago, and also the last thing I've
done in mainline mpd to work inside a thread, go figure.

src/sig_handlers.c

@@ -57,7 +57,6 @@ static void chldSigHandler(mpd_unused int sig)
 			else
 				break;
 		}
-		directory_sigChldHandler(pid, status);
 	}
 }