Broken since commit f8bfea8b

The JACK output plugin would not correctly upmix mono input files when exactly 2 output ports were configured. This fixes that.

The "end of stream" packet is not special; it contains normal data,
and thus we should pass it to OnOggPacket().

This fixes one part of https://github.com/MusicPlayerDaemon/MPD/issues/601

Closes https://github.com/MusicPlayerDaemon/MPD/issues/604

Closes https://github.com/MusicPlayerDaemon/MPD/issues/607

This exception is usually thrown by class DecoderBridge, but the Opus
plugin (ab)uses it as well, so we need to catch it.

MPD used to do that when this code lived in the player thread, but it
was removed by commit 98a7c62d; and
the replacement code in the ALSA output plugin didn't have it.

Without this timer, DispatchSockets() may disable the
MultiSocketMonitor and if Play() doesn't get called soon, it never
gets a chance to generate silence.  However if Play() gets called,
generating silence isn't necessary anymore...

Resulting from this misdesign (added by commit ccafe3f3 in 0.21.3),
the silence generator didn't work reliably.

In DispatchSockets(), when there was not enough data, but enough for
current playback, the method would disable the "active" flag so the
next Play() call would re-enable the MultiSocketMonitor.

This was an abuse of the flag which could result in a crash
in Cancel(), because that method asserts that the period_buffer is
empty, which it may be not.

The solution is to add anther flag called "waiting" which shares some
behavior with the old flag.

Draining isn't finished just because the period_buffer has run empty.
It is only finished after snd_pcm_drain() has succeeded.

Apparently, if snd_pcm_drain() returns EAGAIN, it does not actually
want to be called again; the next call will snd_pcm_drain() will also
return EAGAIN, forever, even though the PCM state has meanwhile
switched to SND_PCM_STATE_SETUP.  This causes a busy loop; to fix
this, we should always check snd_pcm_state() to see if draining is
really required.

See https://github.com/MusicPlayerDaemon/MPD/issues/545

Fix build breakage from commit ea639269

Closes https://github.com/MusicPlayerDaemon/MPD/issues/589

This makes the configuration error more visible, possibly addressing
one part of https://github.com/MusicPlayerDaemon/MPD/issues/589

Commit 1eae9339 added support for
multiple "groups" in the "list" command, and this change allows
clients to detect that this behavior, which had been documented for
several years, is now implemented properly.

Workaround for a regression caused by commit
a06bf388, revealing a problem with
discarding odd numer of frames in the DSD_U32 and DoP converters,
causing distortions with DSD_U32 and DoP on 32 bit CPUs.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/469