Change m_badge_earned into a vector.

This makes it so that getting enough kills to cross the threshold for more than one badge simultaneously grants all of them (with the debriefing showing only the last one) and backs out all of them if the scoring is backed out.

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11198 387891d4-d844-0410-90c0-e4c51a9137d3