Allow defining random-teleport exclusion zones in lua

Adds a new lua command

  des.exclusion({ type = "teleport", region = { x1,y1, x2,y2 } });

which allows defining "exclusion zones" in the level, areas where
random teleports (or falling into the level) will never place the hero.
Does not prevent targeted teleportation into the area.

Breaks saves and bones.

include/decl.h

@@ -340,6 +340,7 @@ struct instance_globals_e {
     struct bubble *ebubbles;
 
     /* new stuff */
+    struct exclusion_zone *exclusion_zones;
     int early_raw_messages;   /* if raw_prints occurred early prior
                                  to gb.beyond_savefile_load */