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

This eliminates a tiny amount of overhead because the compiler can
choose how to pass the parameter.

This fixes the Windows build.  Linking failed because some packages
(e.g. libFLAC) default to enabling `_FORTIFY_SOURCE`, which is broken
in recent mingw versions
(https://github.com/msys2/MINGW-packages/issues/5803).

While libsndfile doesn't like partial reads in the middle of a file
(see commit 95ac6071), it allows partial reads at the end of a file.
It doesn't pay attention to the file size when issuing a read.

Commit ecb67a1e (MPD 0.18.12) was a regression: previously,
partial reads at the end of a file were possible, but switching to
decoder_read_full() made this an error condition.  This way, a portion
at the end of each file was lost, leading to corruption with gapless
playback (https://github.com/MusicPlayerDaemon/MPD/issues/936).

This fix switches to the newly introduced function
decoder_read_much(), which does the same as the code before commit
ecb67a1e.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/936

Our AudioObjectGetPropertyDataT() wrapper throws exception on error,
and calling it from OSXOutput::Disable() can cause MPD crash due to
std::terminate().

Closes https://github.com/MusicPlayerDaemon/MPD/issues/932