The AudioOutput struct (which is exposed to all plugins) should not be
aware that it's being controlled by another thread.

Will move attributes from struct AudioOutput that are specific to the
OutputThread.  The new struct AudioOutputControl is a holder for the
AudioOutput pointer.

This prepares for making the output list more dynamic, to allow moving
outputs to between partitions.

Closes #31

Fixes a potential crash bug which is actually unreachable, because the
"pcm" plugin is never invoked when there is no (matching) MIME type.

After a seek failure, the buffer may still be full, and then
FillBuffer() aborts with assertion failure.

See https://picard.musicbrainz.org/docs/mappings/

This tag is useful when the user would like all releases of the same
album to be sorted next to each other.

upgrade zlib to 1.2.11

An ino_t is usually a 64 bit integer, and some file systems (such as
Linux's kernel NFS client) really uses the upper 32 bit.  This can
lead to false positives in the directory loop detection in
FindAncestorLoop().  Increasing these two attributes (in
StorageFileInfo and Directory) to 64 bit adds little overhead, but
makes the check a lot safer.

See code comment.  Bug was reported against MPD, but it's really an
alsa-lib bug.

 https://bugs.musicpd.org/view.php?id=4662

release v0.20.6