support for custom chunksizes

landrush.get_chunk(pos) -> Without this fix claimed areas shown by hud.lua are .5 too big or too small (the border was in the middle of the blocks).

landrush.get_chunk_center(pos) -> Fix for landrush:showarea position when using a custom chunksize.

Existing landclaims with chunksize>16 may move by 1 block with this changes.

Thanks to Ducky!