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.

include/extern.h

@@ -1815,6 +1815,7 @@ extern int do_play_instrument(struct obj *);
 #if !defined(CROSSCOMPILE) || defined(CROSSCOMPILE_TARGET)
 extern struct selectionvar *l_selection_check(lua_State *, int);
 extern int l_selection_register(lua_State *);
+extern void l_selection_push_copy(lua_State *, struct selectionvar *);
 extern void nhl_push_obj(lua_State *, struct obj *);
 extern int nhl_obj_u_giveobj(lua_State *);
 extern int l_obj_register(lua_State *);