Replaces decoder_data() and others.

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

This is required if a stream ands without another chained FLAC file.

It's always 0.

If the duration is unknown, pass SignedSongTime::Negative(), as
documented for decoder_initialized().

This is obsolete because it has been moved to the MPD core.

libFLAC API documentation suggests that FLAC__stream_decoder_flush()
should be called to recover from FLAC__STREAM_DECODER_SEEK_ERROR.

Stop after all fatal errors.  This fixes assertion failures in
libFLAC.

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

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.