Fixes use-after-free bugs causing assertion failures at shutdown,
because all "abandoned" SocketEvents are still in the linked list.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/986
Closes https://github.com/MusicPlayerDaemon/MPD/issues/987

dup2 returns new_fd on success, not 0

Reduce header dependencies for SocketEvent.hxx.

Not only those which are "ready".

Prepare for adding more code here.

This implements proper rounding, amending commit dcbb9fe0

release v0.22.1

snprintf() is available on mingw, and the libnfs kludge broke the
build with mingw, because sprintf_s() was now both an inline function
and a "dllimport" function (because the macro renamed the inline
function snprintf() to sprintf_s() in mingw's stdio.h).

Fixes regression from commit 1473d847

Fixes regression by commit 23d5a2b8 -
that commit always pretended that any Opus file has both track and
album gain, and thus disabled the fallback to the other if one is not
set.

This patch changes the logic to only submit ReplayGain if at least one
value is set, and apply the offset only to that value.  If none is
available, then the new check in HandleAudio() will submit only the
output gain.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/977

Closes https://github.com/MusicPlayerDaemon/MPD/issues/979