On linux-rt, kernel IRQ threads are configured with priority=50, and
this change configures MPD somewhat below that priority, leaving some
room for other programs to be configured in between.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/643

release v0.21.15

Apparently, libmpcdec sets gain/peak variables to zero if they are not
present.  This clashes with our formula and results in bogus values
which cause noise during playback.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/640

The formula did not consider the channel count.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/639

This commit adds a PlaylistPlugin attribute "as_folder" which for now
is only enabled in the "CUE" playlist plugin (which handles separate
"*.cue" files).  If a playlist with this flag set is being scanned
during database update, it will be parsed and its contents will be
added to the database.  This allows clients to inspect them like
directories and its contents will be searchable.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/39

Will be used for Song objects representing tracks inside a CUE file.

memcmp() can overrun the buffer.

Eliminate unnecessary dynamic allocations.

Don't catch and log exceptions here; let the caller decide.

This attribute is not a URI; it is just the filename without its
parent directory path.  To avoid confusion, let's rename it to
"filename", leaving the struct without a "uri" attribute.