get the operating system from within the OperatingSystem class, and cache the result

git-svn-id: svn://svn.icculus.org/fs2open/trunk@10727 387891d4-d844-0410-90c0-e4c51a9137d3
move mod validation into the OperatingSystem class as well

git-svn-id: svn://svn.icculus.org/fs2open/trunk@10728 387891d4-d844-0410-90c0-e4c51a9137d3