observing monster become invisible

Requested by one of the beta testers 13 months ago... when a visible
monster becomes invisible and vanishes, mark its map location with
the remembered, unseen monster glyph.  (When the player zaps a
monster with a wand of make invisible, that only happens if the wand
type is known.  I'm not sure that's right but didn't alter it....)

The request suggested also doing it for a monster who disappears by
teleporting away, but I haven't attempted to implement that.

src/zap.c

@@ -280,6 +280,7 @@ struct obj *otmp;
         mon_set_minvis(mtmp);
         if (!oldinvis && knowninvisible(mtmp)) {
             pline("%s turns transparent!", nambuf);
+            reveal_invis = TRUE;
             learn_it = TRUE;
         }
         break;