implement the deterministic mode for sp.

the player can now choose whether he want to play a new SP game in the
deterministic mode where he'll get the same results when he reloads a
game.

i don't really know much about the gamestatus cpp/hpp file, so i just
copied from the difficulty code.

i think it would also be nice to enable the deterministic mode for MP
but i currently don't know enough about the mp connect code to that.