enable wesnoth.mp_settings.active_mods and side_users, using string join
functions as to_config() does.
int mp_countdown -> bool, oops
delete explicit attribute references, write a cfg instead
moved mp_settings to be a (conditional) child of game_config,
using return_cfgref_attrib fcn.
works fine in testing, using path wesnoth.game_config.mp_settings
removed now unnecesary include from lua.cpp
minor rearrangement, prefer to use local variable