See http://bugs.musicpd.org/view.php?id=4496

See http://bugs.musicpd.org/view.php?id=4485

MPD does not really take advantage of memory-mapped I/O by generating
data right into the ALSA buffer; using plain snd_pcm_mmap_writei() has
no advantage compared to snd_pcm_writei().  Let's kill this
non-feature.

Fixes the --disable-dsd build.

See http://bugs.musicpd.org/view.php?id=4498

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

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

Translate SampleFormat::DSD to SND_PCM_FORMAT_DSD_U8, which was added
to alsa-lib 1.0.27.1.

Make it "const".