Fix for Mantis 3093: mn.getMissionFilenname() doesn't return a value if in GS_STATE_BRIEFING

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11077 387891d4-d844-0410-90c0-e4c51a9137d3
Parts of mantis 3103 (From MageKing17)

Points 3-5 from the mantis ticket
i.e. use OPF_SSM_CLASS in appropriate places

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11078 387891d4-d844-0410-90c0-e4c51a9137d3