mimics vs DUNGEON_OVERVIEW (trunk only)

Noticed while looking into whether I could use DUNGEON_OVERVIEW data
for something useful, it was recording accurate terrain type for locations
covered by mimics who were mimicking furniture (such as stairs or altars).
Hero should remember the fake terrain rather than whatever is actually
underneath the mimic.

     No fixes entry; user-contributed DUNGEON_OVERVIEW is post-3.4.3 code.

include/extern.h

@@ -1282,6 +1282,7 @@ E struct permonst *NDECL(courtmon);
 E void FDECL(save_rooms, (int));
 E void FDECL(rest_rooms, (int));
 E struct mkroom *FDECL(search_special, (SCHAR_P));
+E int FDECL(cmap_to_type, (int));
 
 /* ### mon.c ### */