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().

Move code from replay_gain_get_real_mode().