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.
Instead of just plain old boring mazes, spice up Gehennom by
occasionally adding lava, iron bars, or even mines-style levels
(with lava, of course).
Of the fixed Gehennom levels, only Asmodeus' lair has been changed
to add some random lava pools.
Also some lua fixes and changes:
- Fixed a selection negation bounding box being wrong.
- Fixed a selection negated and ORed returning wrong results.
- des.map now returns a selection of the map grids it touched.
- When using des.map contents-function the commands following the
map are not relative to it.
