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.

src/trap.c

@@ -2570,8 +2570,9 @@ blow_up_landmine(struct trap* trap)
 {
     int x = trap->tx, y = trap->ty, dbx, dby;
     struct rm *lev = &levl[x][y];
-    schar typ;
+    schar old_typ, typ;
 
+    old_typ = lev->typ;
     (void) scatter(x, y, 4,
                    MAY_DESTROY | MAY_HIT | MAY_FRACTURE | VIS_EFFECTS,
                    (struct obj *) 0);
@@ -2607,6 +2608,7 @@ blow_up_landmine(struct trap* trap)
             }
         }
     }
+    spot_checks(x, y, old_typ);
 }
 
 static void
@@ -5996,4 +5998,20 @@ ignite_items(struct obj* objchn)
     }
 }
 
+void
+trap_ice_effects(xchar x, xchar y, boolean ice_is_melting)
+{
+    struct trap *ttmp = t_at(x, y);
+
+    if (ttmp && ice_is_melting) {
+        if (ttmp->ttyp == LANDMINE || ttmp->ttyp == BEAR_TRAP) {
+            /* landmine or bear trap set on top of the ice falls
+               into the water */
+            int otyp = (ttmp->ttyp == LANDMINE) ? LAND_MINE : BEARTRAP;
+            cnv_trap_obj(otyp, 1, ttmp, TRUE);
+        } else {
+            deltrap(ttmp);
+        }
+    }
+}
 /*trap.c*/