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).

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