add valid() function in saved_game class

and add comments about other functions.
fix commandline mp start
ignore allow_new_game for reloaded games

this fixes a bug when reloaded games  started immideately for joining
players.
expand save_id default value eralier

Set this default value immideately after reading the scenario is
importent because otherwise we might endup settings this value to the
multiplayer players name, which would break carryover. (doing this in at
config loading in game_config would be ok too i think.)
fix mp reloading

having with network/network_ai together with force_lock_settings_ cause
assertion errors because update_controller_options won't add any
possible options.

this patch restores the previous behaviour where force_lock_settings_
was ignored for reloaded games.