melting ice timeout issues

Reported directly to devteam by entrez via email:
>
> I noticed some potential issues with (melting) ice:
>
>   * Digging down into ice, or setting a land mine on the ice and
>   triggering it, doesn't remove the melt_ice timeout, so it can result
>   in a sequence like dig down -> pit fills with water -> freeze water
>   -> freezing water tries to set melt_ice timeout -> duplicate timeout
>   impossible.  Or if you don't freeze the water again, melt_ice will
>   run on a non-ice surface, which might at least produce strange
>   messages.
>
>   * Setting a land mine on ice: melting ice doesn't do anything with
>   the trap, so there is still a land mine which you can trigger by
>   flying over the water (the land mine's trigger is also still
>   described as being 'in a pile of soil', despite being underwater at
>   this point).  Similar thing happens with bear traps.
>
>   * Not really related to _melting_ ice, but an exploding land mine
>   doesn't reset the typ from ICE to FLOOR (like normal digging does),
>   so it will result in a square with a pit that is also an ice square,
>   where the ice can melt under the pit and produce a combination
>   pit/moat.  If you then freeze the moat, the pit reappears on top of
>   the ice.

doc/fixes37.0

@@ -646,6 +646,8 @@ breaching a shop wall, using locking magic to put a door there, then unlocking
 	that door yielded a situation where subsequent shop damage repair
 	produced invalid map data which resulted in an impossible() warning
 	about "wall_angle: unknown" during map display
+melting ice timer could persist after the ice was gone from digging or from an
+	exploding land mine
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository