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.

test/test_des.lua

@@ -243,9 +243,9 @@ III]] })
 ...]], contents = function(map)
                 des.terrain(0,0, "L");
                 des.terrain(map.width-1,map.height-1, "T");
+                check_loc_name(0, 0, "lava pool");
+                check_loc_name(2, 2, "tree");
    end});
-   check_loc_name(0, 0, "lava pool");
-   check_loc_name(2, 2, "tree");
 end
 
 function test_feature()