Level temperature

Allow setting a per-level "temperature": hot, cold, or temperate
via special level flags. Currently it only affects some messages
in Gehennom, but it could be expanded to ice melting, water freezing,
or monster generation, for example.

Invalidates saves and bones.

doc/lua.adoc

@@ -532,6 +532,9 @@ Set flags for this level.
 | noflip        | Prevent flipping the level.
 | noflipx       | Prevent flipping the level horizontally.
 | noflipy       | Prevent flipping the level vertically.
+| hot           | Level is hot. Dungeon flag "hellish" automatically sets this.
+| cold          | Level is cold.
+| temperate     | Level is neither hot nor cold.
 |===
 
 Example: