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

Check opus_decoder!=nullptr instead.

After seeking, the MPD core automatically refreshes the timestamp, and
thus discards the value from decoder_timestamp().

Reduce header dependencies.

Remove the "whence" parameter that is not actually necessary, and only
complicates the InputStream implementations.

Parse the R128_TRACK_GAIN comment string.

Call decoder_timestamp().  This is not necessary currently, but will
be as soon as we implement seeking.

This requires seeking to the end-of-stream, checking its granulepos,
and then seeking back to the previous file position.  We do this only
for local files.