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

The TAG_MODIFIED handler (i.e. playlist::TagModified()) works only if
the modified song is the current song - something that is not updated
until SYNC_WITH_PLAYER is finished.  This fixes tag updates right
after a new song is started.

https://bugs.musicpd.org/view.php?id=4665