Re-implement XClientMessage handling so that we can feed screen_find and

client_find valid resources as needed, relieving the need for
screen_find to ungracefully handle invalid root windows. Removes a long
standing XXX.  Should theoretically allow XClientMessage handling on
more than one X screen.  Alter callers of screen_find to handle
failures.

screen.c

@@ -100,8 +100,8 @@ screen_find(Window win)
 		if (sc->rootwin == win)
 			return(sc);
 	}
-	/* XXX FAIL HERE */
-	return(TAILQ_FIRST(&Screenq));
+	warnx("screen_find failure win 0x%lu\n", win);
+	return(NULL);
 }
 
 void