Fix check_victory (for mattsc)

*** Please don't merge this, this is in a testing and review stage. ***

This branch is intended to address concerns raised by mattsc on irc. See http://www.wesnoth.org/irclogs/2014/03/%23wesnoth-dev.2014-03-26.log
and the day before for discussion.

The proposed solution is to introduce a field of [side] "fight_on_without_leader",
and refactor check_victory so that any side with this field true and a unit alive
is not considered "defeated". We use this instead of no_leader in places where
that was being used (erroneously?)