aout: split aout_PrepareStereoMode

 - aout_PrepareStereoMode() prepares the "stereo-mode" var list and return the
default mode.
 - aout_HasStereoMode() returns true if a mode is available (in "stereo-mode" var list)
 - aout_UpdateStereo() does the actual changes.

No functional changes. This will be used by next commits.