Keep track of whether there is data being played, and don't call
AudioOutput::Drain() after Cancel() has been called already.

In this state, we can't make any progress.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1084

Drain only if it was requested explicitly.

It is useless, because we're always checking pa_stream_is_corked().

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1022

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1031

Commit 79b23663 added the field `skip`
to support unaligned reads, but set the `offset` field to a wrong
value.  This resulted in miscalculation of `remaining`, causing
an assertion failure.

The fix is to assign `offset` the correct value, but consider the
`skip` value in the assertion.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1067

After a Seek() to an odd offset, some data needs to be skipped from
the start of the block, and reading right into the given buffer
doesn't work.

Conductor was incorrectly saved to Performer tag in MPD database

Fixes Windows compatibility.

The Google overlords require me to change to 29 or else I can't upload
new releases to Google Play.

 https://developer.android.com/distribute/best-practices/develop/target-sdk

Increasing the protocol version to 0.22.4 to allow clients to detect
this feature.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1038

Of course, it should do that!

I havn't yet figured out how to use Android's system CA certificates
with CURL/OpenSSL, so a temporary workaround is to disable verify_peer
by default.  The data MPD transfers isn't extremely important, so the
servers's authenticity isn't extremely important either.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1059

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1048

This properly prints the "Time"/"duration" values for songs in virtual
CUE folders.

This is loosely related to
https://github.com/MusicPlayerDaemon/MPD/issues/1048

`PRIoffset` was wrong, because it expects an `offset_type`
(i.e. `uint64_t`).  This broke on 32 bit machines where `size_t` has
32 bits.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1058

"The documentation for mpd.conf still links to https://replaygain.org
– according to archive.org, this domain has been defunct since
2014-09-14, shortly after got domain squatted and ever since hosts
dubious content."

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1050