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

Basic definition to solve http://bugs.musicpd.org/view.php?id=3147 and
http://bugs.musicpd.org/view.php?id=3255

This systemd feature has been removed a while ago without replacement,
and it turns out that systemd developers suggest not using control
groups at all to assign real-time privileges.  Therfore, a replacement
feature will not be implement in future systemd releases, and we can
really remove those lines completely.

See http://bugs.musicpd.org/view.php?id=4413

Fixes http://bugs.musicpd.org/view.php?id=4432