Rename struct Encoder to PreparedEncoder, and add a new (abstract)
class Encoder which represents one encoder instance.

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 only applies to linux systems.  Here, sched_setscheduler() is
called to get realtime scheduling.  With this patch, the return value
of this function is now checked and a warning / error message is
generated if it fails.

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

Fixes mixer lag (http://bugs.musicpd.org/view.php?id=4425).

Also uses the notification system to display tags.