Now there is only one single entry point for both, instead of previously 2 entry and 4 exit points.
This also eliminates the explicit shutdown of ZMusic. Timidity++'s two buffers have been put in containers that self-destruct on shutdown and calling dumb_exit is not necessary because the only feature requiring it is not used by any code in the music library.
Also made the decode_vorbis function in DUMB a function pointer so that the library does not depend on high level code and can just ignore the vorbis case if no supported.
