a check for deterministic mode in mp.

we now check during the start event whether the game is a mp game which
runs in deteministic mode and give an error in this case. This case
shouldn't cause OOS but it allows players  to cheat. I don't do it
earlier because i cannot use the screen during prestart events, and i
don't know about the code earlier (mp_connect).
remove the 'only_attacks' random mode

i came to the conclusion, that the lag for asking the server for a new
seed is neglectable. So we don't need this mode.