added a sexp to pause and unpause music that is currently playing

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11183 387891d4-d844-0410-90c0-e4c51a9137d3
it's good to check whether the music is paused, for bulletproof-ness

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11184 387891d4-d844-0410-90c0-e4c51a9137d3
when the game returns from a paused state, don't unpause the music paused by sexp

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11185 387891d4-d844-0410-90c0-e4c51a9137d3