fix spawn protection not being correctly loaded.

- mark Chunk as modified when spawn protection is changed
- avoid integer division rounding error
- create one private method to update spawn protection

- changed GSP command to allow entering chunk coordinates

point 2 of http://terrafirmacraft.com/f/topic/7089-spawn-protection-bugs/#entry94989
Merge pull request #699 from CHeuberger/fixSpawnProtectionNotRestoredCorrectly

fix spawn protection not being correctly loaded/saved.