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.

src/decl.c

@@ -342,6 +342,7 @@ const struct instance_globals_e g_init_e = {
     /* mkmaze.c */
     UNDEFINED_PTR, /* ebubbles */
     /* new */
+    NULL, /* exclusion_zones */
     0,      /* early_raw_messages */
     TRUE, /* havestate*/
     IVMAGIC  /* e_magic to validate that structure layout has been preserved */