Similar to commit 32380d1d, these are
the final parts for really fixing
https://github.com/MusicPlayerDaemon/MPD/issues/469

These options are exclusive.

First part of the "real" fix for
https://github.com/MusicPlayerDaemon/MPD/issues/469

The unit test was wrong as well.  D'oh!

Move this sample rate fixup from the ALSA output plugin to PcmExport,
where it belongs.

DSD_U32 packs four bytes instead of one large "sample", thus the
sample rate is one quarter of the input sample rate.  This fixes a
rather critical DSD_U32 playback problem.

Prepare for DSD sample rate fixups.

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

The standard has been renamed since the early draft that was
implemented in MPD.