allow vengeance against nymphs except for specific case

From: "Ken Arromdee" :
> My point is that you should be allowed to take vengeance on thieving
> nymphs too.  The reasoning "a real knight wouldn't kill a nymph for stealing"
> doesn't make sense because the things a real knight would do instead (like
> arresting) aren't part of the game.

This is a compromise.  This doesn't allow vengeance when you were
told "you gladly hand over ...", but does for most other cases, and for
leprechauns.

src/mhitu.c

@@ -1236,11 +1236,6 @@ do_stone:
 			if (!is_animal(mtmp->data) && !tele_restrict(mtmp))
 			    rloc(mtmp);
 			if (is_animal(mtmp->data) && *buf) {
-			    /* set mavenge bit for animals so knights won't
-			       suffer an alignment penalty during retaliation;
-			       note that only happens when the thief succeeds
-			       in getting something (*buf != 0) */
-			    mtmp->mavenge = 1;
 			    if (canseemon(mtmp))
 				pline("%s tries to %s away with %s.",
 				      Monnam(mtmp),