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.

doc/lua.adoc

@@ -528,6 +528,18 @@ Example:
  des.engraving({x,y}, "engrave", "Foo");
 
 
+=== exclusion
+
+Exclude an area of the map from being randomly chosen target when
+falling or teleporting into the level.  Multiple exclusions per level are allowed.
+
+* type is one of "teleport", "teleport-up", or "teleport-down".
+
+Example:
+
+ des.exclusion({ type = "teleport", region = { 0,0, 10,5 } });
+
+
 === feature
 
 Create a feature, and set flags for it.