savegame: Add load_state(), save_state() to load/save any format.

load_state() will try to detect the format to load, save_state() will save to the current format.
game: Move initialization of game_t to game_init() and related.

Add functions game_load_random_map() and game_load_save_game() to complete the initialization after game_init() has been called.
Move serf_animation_table to interface.