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

[atomicdryad]

• 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& 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.

[CIB]

Everything checks out IMO. I can merge this once you give the OK.