fix era carryover for sp

firstly, state_.mp_settings() wasn't being written to config for sp (and therefore wasn't being restored in level_to_gamestate()).
also, I've edit playcampaign.cpp so that sp uses the mp (IO_SERVER) code between scenarios (using connect_engine).
I've disabled the mp_connect screen between sp scenarios for now, but it may be useful to add it as an advanced option later.