ogg: Separate ogg from vorbis code.

ogg: Separate ogg from vorbis code.

This moves vorbis-independent code from ogg_afh.c to the new file
ogg_afh_common.c so that it can be used also for the speex audio
format handler that is added by subsequent patches.

The patch introduces a generic callback structure which contains a
function pointer used to call back into the vorbis code once for each
of the three ogg packets of the audio file header.