Show level annotation when overwriting existing one

Idea by Chris Smith, via UnNetHack

src/dungeon.c

@@ -1867,7 +1867,12 @@ donamelevel()
 
     if (!(mptr = find_mapseen(&u.uz))) return 0;
 
-    getlin("What do you want to call this dungeon level?", nbuf);
+    if (mptr->custom) {
+	char qbuf[BUFSZ];
+	Sprintf(qbuf, "Replace annotation \"%s\" with?", mptr->custom);
+	getlin(qbuf, nbuf);
+    } else
+	getlin("What do you want to call this dungeon level?", nbuf);
     if (index(nbuf, '\033')) return 0;
     (void)mungspaces(nbuf);