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.

doc/lua.adoc

@@ -544,7 +544,9 @@ Example:
 === map
 
 Construct a piece of the level from text map. Takes one parameter, either a text string
-describing the map, or a table with multiple parameters.
+describing the map, or a table with multiple parameters. Returns a <<_selection>> where
+the map locations were put down on. If a contents-function is used, the commands following
+the map are not relative to it.
 
 [options="header"]
 |===
@@ -570,6 +572,7 @@ Example:
    des.terrain(0,0, "L");
    des.terrain(map.width-1, map.height-1, "T");
  end });
+ local sel = des.map([[LLL]]);
 
 === mazewalk