add functionality for define= in [scenario] tag

Added a new public data member, scenario_define, to class game_classification
(alongside campaign_define). Added a new public method, prepare_for_scenario,
to multiplayer create_engine. This method is called from process_event in
multiplayer_create similarly to the existing prepare_for_campaign method.
The prepare_for_scenario method reads a define attribute from a scenario
tag into classification.scenario_define, which is then processed by
load_game_config_for_game. The effect of this change is that, if all
scenarios use define= to isolate sections of their code, much less config
data will be loaded initially. Config data is reloaded upon scenario
selection, with isolated content for the selected scenario included.