release v0.19.20

This commit changes a minor queue priority design to something which
makes a little bit more sense.

Previously, a song that had already been played would only be
re-enqueued if its priority had just been raised above the current
song's.  This means that if it was already above, it was not
re-enqueued.  That is a surprising behavior, because users expect a
song to be played when its priority is raised.

Now the song is always re-enqueued if its priority is raised (and
above the current song's - no matter if it has already been above
before).

 https://bugs.musicpd.org/view.php?id=4592

e.g. when running 'update-rc.d mpd disable'

It's wrong to use std::system_category() for both GetLastError() and
errno on Windows.  Apparently, everybody uses std::generic_category()
for errno values, which appears to be a safe choice.

Some discussion on this confusing topic can be found here:

 https://stackoverflow.com/questions/28746372/system-error-categories-and-standard-system-error-codes

The Windows standard library emulates part of POSIX, which includes
errno.

Obsoletes the same variable from AudioConfig.cxx.

Remove dependency on ReplayGain global variables.

 https://bugs.musicpd.org/view.php?id=4595

Don't use the global variables in class DecoderBridge; instead,
forward these values to the decoder thread via PlayerControl and
DecoderControl.

Move code from ReplayGainConfig.cxx.

Declare two named elements.  An enum should not be used as an array
index, as this is error prone.

Eliminates Complete().