Eliminate some duplicate code.

This fixes a spurious "single" mode bug which occurs when using "play"
or "seek" to start playback on the song that is currently paused: in
that case, the main thread never queues the next song, and at the end
of the song, the player thread exits Run(), stopping playback, and
after that, the main thread starts the next song without considering
"single" mode.

By calling OnPlayerSync(), we ensure that the main thread gets a
chance to queue the next song before the player thread exits the Run()
loop.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/850

The log levels have always been very confusing (and badly named), but
this was most confusing: if there's a log level called "default", why
is it not the default?

Closes https://github.com/MusicPlayerDaemon/MPD/issues/926

FFmpeg implements RTSP as a demuxer, not as a protocol handler.  Thus,
avio_open() cannot be used, and our input plugin cannot handle RTSP.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/930

Oh the horror!  This plugin cannot possibly ever have worked.  It was
broken from the start, when it was added in commit 37796699 nearly
twelve (!) years ago.

The plugin would always read at sector boundaries, so it could only
ever work at multiples of 2 kB.

This makes the unit test fail.  D'oh!

Don't silently return 0 when there is no more data, because this may
crash the caller.  And flush output even if input EOF has been reached.

Sometimes, zzip_file_read() returns 0 even though the end of the file
was not reached.  This causes assertion failures in
DecoderBridge::Read().

Closes https://github.com/MusicPlayerDaemon/MPD/issues/935