Also simplified the sound init decision making. With FMod gone there is no reason to be pedantic here. Even the check of snd_backend for the Null device could be omitted here, its only realistic use is '-nosound'.
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.
FMod had MP3/Ogg playback integrated right into itself, and the OpenAL backend tried to replicate this functionality.
This functionality, however, has been removed over two years ago when FMod started breaking things more and more, it was only this backing implementation that was left in.
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.
