Closes https://github.com/MusicPlayerDaemon/MPD/issues/846

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!

MPD uses soxr with prefined resample recipes. Soxr also support defining a recipe your self.
This commit will support a custom recipe by changing the existing quality setting to "custom".

The same structs as the predefined recipes uses can now set by hand.

This will make the following settings available:
- precision 16|20|24|28|32 bits, example "28"
- phase_response - 0-100, example "45"
- passband_end - used bandwidth of source 80-99.7%, example "99.7.0"
- stopband_begin - anti aliasing 100.0+%, example "100".
- attenuation - signal reduciton in dB's, 0-30. example "3.0".
- flags "0" - additional bitmask with extra settings

The data is set in the structs soxr_quality_spec and soxr_io_spec (found in soxr.h).