map <=> abs coordinate helper functions, long distance generation, add_item oob

* map: load / save / generate will properly handle areas beyond game->cur_om,
   load accepts optional *overmap argument to allow creation of additional
   long distance map areas ( for nukes and such ).
 * map::loadn: don't iterate vehicles if update_vehicles==false. Skip spoilage check too
 * submap: store absolute submap coordinate (as in maps.txt) as x,y,z
 * map: store absolute submap coordinate of top-left submap as point abs_sub
 * map: getabs(x, y) converts local-to-map tile coordinates to absolute tile
   coordinate (ie, if cur_om() = -1,0 and levx = 1,2, getabs(0,-500) will return
   -4308,-476
 * map: getabs: [TEMPORARY/TESTING] perform sanity check that grid[0] and abs_sub agree.
 * map: inboundabs(x, y): works with absolute tile coordinates instead of local-top-map
   coordinates
 * map: getspawns(x,y); returns std::vector<spawn_point>& for the appropriate submap
* map::add_item has optional argument to add to OOB coordinates, by calling
  map::add_item_anywhere(x, y, z, item, maxitems) which adds to absolute coordinates.