Revert commit 70495aad by rewriting it.  Turns out, in old FFmpeg
versions, copying the AVPacket is necessary.

Don't scan tag items if the handler doesn't implement the tag()
method.

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This is obsolete because it has been moved to the MPD core.

Referencing the attribute "partition" is illegal after Close(),
because Close() deletes "this".

Fixes use-after-free bug (https://bugs.musicpd.org/view.php?id=4548).

The "seeking" flag is not set for the initial seek, and so
decoder_read() could be canceled when another SEEK was emitted during
initial seek.

This fixes several seek problems, for example the one reported for the
FLAC decoder plugin:

 https://bugs.musicpd.org/view.php?id=4552

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.

No change for regular PCM, but DSD uses 0x69 now.

Fixes abort after seeking on fast machines.

Previously, volume was applied twice: once by PcmVolume, and again by
the hardware mixer.

.. instead of doing it after seeking.  After seeking, the command had
no effect, because CheckDecoderStartup() waits for all outputs to
finish.  This caused a very long delay while seeking and switching
songs (https://bugs.musicpd.org/view.php?id=4534).

This is a kludge which may break system include path order, see
https://bugs.musicpd.org/view.php?id=4524

See https://bugs.musicpd.org/view.php?id=4523