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).

--stdout was renamed to --stderr in 72617395.

install_man() is currently broken with Meson and doesn't support a
custom target argument.

The problem with this kludge is that both mpd.1 and mpd.conf.5 are
installed in /usr/share/man/man1/, but apparently, there's no solution
yet.

Build with `rst2man mpd.1.rst mpd.1`

This allows automatic optional detection of Sphinx.  This will be
useful when we start building the manpages with Sphinx, which many
users may want to have.

Currently, the hardcoded URL points to a non-existent page.

Fixup after e4dad42c

This per-object Mutex replaces the global `smbclient_mutex`.

This is no longer necessary with the new API.

This is no longer necessary with the new API.

Preparing to replace `smbclient_mutex`, for finer-grained locking.

We no longer need to call smbc_init() because we don't need the compat
layer anymore.

As a side effect, the input plugin closes the SMB/CIFS connection
after closing the file.

This solves one part of
https://github.com/MusicPlayerDaemon/MPD/issues/916