Disable start of scenario saves in multiplayer

Relevant irc logs: http://www.wesnoth.org/irclogs/2014/11/%23wesnoth-dev.2014-11-12.log
http://www.wesnoth.org/irclogs/2014/11/%23wesnoth-dev.2014-11-14.log

Tested with SigurdFD's random campaign, further testing would be appreciated.