Allow missions to exit to mainhall on successful completion

Activate with either a mission flag, or the 3rd parameter to the
end-mission sexp. Covers ending via the debrief, skipped from dying
5x times or skipped training mission
Does NOT work for redalert missions or multi (yet)

git-svn-id: svn://svn.icculus.org/fs2open/trunk@10832 387891d4-d844-0410-90c0-e4c51a9137d3