release v0.21.6

Closes https://github.com/MusicPlayerDaemon/MPD/issues/465 and
https://github.com/MusicPlayerDaemon/MPD/pull/466

And disable libvorbis detection if Tremor was explicitly enabled.

This fixes a crash bug caused by libvorbis/Tremor ABI conflict caused
by commit 4f7d52db

Previously, MPD would skip the current song after attempting to seek
to its beginnig, because that was a seek to offset 0.  At offset 0,
MPD will see the BOS packet again, which results in throwing
StopDecoder in MPDOpusDecoder::OnOggEnd().

Closes https://github.com/MusicPlayerDaemon/MPD/issues/470

Unlike pa_channel_map_init_auto(), pa_channel_map_init_extend() does
not fail if there is no valid mapping for the given channel count, but
instead maps additional "AUX" channels.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/493

The `tag_builder.empty()` check was wrong for the SubmitReplayGain()
call.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/497

Clarification for https://github.com/MusicPlayerDaemon/MPD/issues/505

The old implementation didn't make a lot of sense; the "!=" operator
was not actually the opposite of "==".

Closes https://github.com/MusicPlayerDaemon/MPD/issues/505

A few of those tests fail due to bugs.

There is a minor new feature (commit 713c1f2b) and clients might be
interested in detecting it by the protocol version.

The "NN" suffix used to mean "no negation", but that's not how it's
implemented today.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/507

Disable the 0.11 compatibility mode if the only argument is a filter
expression.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/506

Closes https://github.com/MusicPlayerDaemon/MPD/issues/508