fix #H177 - mimicking gold while hallucinating (trunk only)

From a bug report, having hallucination time out while
mimicking an orange (instead of gold, after eating a mimic corpse), you'd
still get the hallucinatory end-of-mimicking message about not wanting to
be peeled.  If hallucination state is toggled, update the pending message
and change the hero's appearance.  In practice, only the orange-to-gold
case can occur.  Anything which might trigger gold-to-orange will have
terminated the hero's mimickery befort that happens.

include/extern.h

@@ -567,6 +567,7 @@ E int NDECL(eatfood);
 E int NDECL(opentin);
 E int NDECL(unfaint);
 #endif
+E void NDECL(eatmupdate);
 E boolean FDECL(is_edible, (struct obj *));
 E void NDECL(init_uhunger);
 E int NDECL(Hear_again);