Replaces decoder_data() and others.

Replaces decoder_initialized().

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

Prepare to reuse the same code for a new Vorbis decoder plugin based
on libvorbis instead of libvorbisfile.

Initialize the OggStreamState as soon as the first page is seen; no
special code needed in the caller.

Required for OpusTags packets which contain artwork.

See https://bugs.musicpd.org/view.php?id=4520

The old struct config_param remains only for top-level string options.