Unify sound API configuration.

Unify sound API configuration.

'speaker_backend' is now spelled 'api' (though the old name still
works) and effects the choice of volume setting API as well as the
device used by the speaker process.

For volume setting, only OSS is implemented (still).  The new
structure means that ALSA volume setting no longer works as it will no
longer use the OSS interface instead.  However, this was never
reliable as it depended upon ALSA's OSS emulation being enabled.