hiding under a cockatrice corpse

Reported a while back, a (stonable) hiding monster will hide at a location
containing only a cockatrice corpse.  While it would be interesting to
allow monsters to try, and stone themselves as a result, I chose the
simpler fix which is to not have monsters hide in such situations.  I found
the hiding code was duplicated in several places, so I moved it into a new
hideunder() function that works for both the hero and monsters.

src/invent.c

@@ -2506,7 +2506,7 @@ long numused;
 	}
 	delobj(otmp);
 	if (at_u && u.uundetected && hides_under(youmonst.data))
-	    u.uundetected = OBJ_AT(u.ux, u.uy);
+	    (void) hideunder(&youmonst);
 }
 
 /*