Use walk!=nullptr instead.

Prepare for moving idle events to class Partition.  Right now, it's
just a wrapper for idle_add().

This is necessary to avoid an assertion failure in UpdateQueuedSong()
after LockSeek() has failed.

Don't need to clear the caller-specified one.

playlist_domain is no longer used for class Error.

Improved exception-safety.

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