Some FFmpeg filters change the sample format, and since MPD assumes
this never happens, this results in loud noise instead of music.  This
commit finally implements the TODO comment by sending one frame of
silence to the filter and checking the output frame's format.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1009

Fixes #994 - moveoutput: new AudioOutputControl created from copyMoving an output to a partition is now done via MultipleOutputs::AddCopy(),using a new AudioOutputControl constructor. Tags and always_on settings willpersist when moving outputs between partitions.

Have similar information about config file in both mpd.1 ans mpd.conf.5

Update description with content from user manual [0].
Remove obsolete information regarding "required parameters" and log_level

[0] https://www.musicpd.org/doc/html/user.html#the-configuration-file

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1001

Pass a std::string to PathTraitsUTF8::Relative(), implicitly casting
it to std::string_view.  This selects the right overload which returns
std::string_view instead of `const char *`; the latter could return
`nullptr` which would cause the implicit conversion of the return
value to std::string_view to crash.

Regression caused by commits ead20898 and a98d627c.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/995

This adds support for input samples other than 16 bit to the FFmpeg
filter plugin.

If no conversion is necessary, return the child Filter as-is.  This
allows removing all nullptr checks from AutoConvertFilter.

If the FFmpeg filter outputs planar data, interleave it, just like the
FFmpeg decoder plugin does.

To be reused by the FFmpeg filter plugin.

This compatibility macro has been removed from FFmpeg long ago.

filter_chain_parse() passes a temporary string pointer which results
in a use-after-free in the PreparedChainFilter::Child::Open() error
message.

This allows users to disable the "CUE files as directories" feature
without having to disable the CUE playlist plugin completely.  This
feature has been annoying some users.

It's been 9 years already, and there's no point in insisting on the 21
year old C standard.  MPD doesn't have a lot of C code left, but why
not compile it with the latest language revision.

Since MPD is a long-running daemon, it doesn't make sense to use
dynamic binding.  That allows the relocations to be read-only
("relro"), which a hardening feature.