More interesting Gehennom levels

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.

dat/dungeon.lua

@@ -91,6 +91,7 @@ dungeon = {
       base = 20,
       range = 5,
       flags = { "mazelike", "hellish" },
+      lvlfill = "hellfill",
       alignment = "noalign",
       branches = {
          {