Fixes 2713 - "Freelook Mode Unavailable."

Freelook has been re-enabled with a minimal amount of changes.

Efforts to add in slew control keys as well as the ability to map the mouse to a slew axis have resulted into a much larger project than previously expected. As such, these changes will be postponed until fully complete.

git-svn-id: svn://svn.icculus.org/fs2open/branches/antipodes@10566 387891d4-d844-0410-90c0-e4c51a9137d3