The state ERROR can only be entered during decoder initialization, and
Seek() may only be called after initialization has been finished.

Some optimized implementations of memcmp() may not start from the
beginning of the string, and may thus segfault.

If the decoder was just started, it already seeks to the desired
position.

If this gets called while seeking, do the right thing.  This moves the
elapsed_time setter from method Run(), which is now redundant.

Allows removing a few special cases in the player thread.

It is futile to check for decoder errors before the decoder has
finished startup.  At this time, it's unlikely that the decoder has
already failed.

.. and fix its API documentation.

Assigning nullptr to next_song may disrupt the player thread and
render undefined behavior.

Move ResetCrossFade() to where cross-fading would actually break, to
make the cross-fading state more reliable.

This new library is integrated in the PcmExport class and (if enabled)
converts MPD's channel order (= FLAC channel order) to ALSA channel
order.

This fixes:
 http://bugs.musicpd.org/view.php?id=3147
and
 http://bugs.musicpd.org/view.php?id=3255