Implement define= for mp scenarios and eras

Implemented reloading of configs with define= attributes in both mp_create and mp_wait::join_game().
Added myself to about.cfg and updated changelog.
Add define functionality to modifications.

Added a new method, void create_engine::prepare_for_era_and_mods(),
which sets era and mod in game classification.
Prepare_for_[scenario, campaign] then set their respective defines and reload.