part3 of 3 for GitHub issue #1441

> Perhaps related: when a wand of sleep hits a disguised already-sleeping
> mimic (about which it is a separate question if they should go into
> disguise when sleeping, is it supposed to be automagic or conscious
> effort for them? but I digress), the mimic is not revealed (should it?)
> but the message says "hits a mimic".

Adjust restrap() so that a revealed mimic won't disguise itself while
sleeping. This seems to be in keeping with mimic lore.

Also, normal shop sounds (chime of register etc.) will wake a
mimic up from indeterminate sleep.

Closes #1441

src/mon.c

@@ -4647,6 +4647,13 @@ restrap(struct monst *mtmp)
         return FALSE;
 
     if (mtmp->data->mlet == S_MIMIC) {
+        if (mtmp->msleeping || mtmp->mfrozen) {
+        /*
+         * The mimic needs to be awake to disguise itself
+         * as something else.
+         */
+            return FALSE;
+        }
         set_mimic_sym(mtmp);
         return TRUE;
     } else if (levl[mtmp->mx][mtmp->my].typ == ROOM) {