make localization more robust with additional checks on string length, more consting, and stronger typing
and make XSTR const, since we don't want to modify the strings it translates
and then CONST ALL THE THINGS

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@9809 387891d4-d844-0410-90c0-e4c51a9137d3