'cd next', 'cd prev' commands. 'cd play' plays track 1.

git-svn-id: svn://svn.code.sf.net/p/quakespasm/code/trunk/quakespasm@950 af15c1b1-3010-417e-b628-4374ebc0bcbd
cd_sdl: prev and next commands must handle data tracks properly.

git-svn-id: svn://svn.code.sf.net/p/quakespasm/code/trunk/quakespasm@951 af15c1b1-3010-417e-b628-4374ebc0bcbd