Consolidate secondary buffer list processing and locking.

Consolidate secondary buffer list processing and locking.
Two new functions DSOUND_AddBuffer and DSOUND_RemoveBuffer
have been added and the locking has been moved into these functions.
Renamed IDirectSound lock to buffer_list_lock.
Removed redundant test for IID_IDirectSoundNotify and
IID_IDirectSoundNotify8 because they are the same.
Removed check for invalid buffers on list because that shouldn't
happen anymore because of fixed 3d buffer reference counting.
Minor code cleanups.