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.

The bug https://bugzilla.samba.org/show_bug.cgi?id=11413 makes MPD
crash after at most a minute of using the plugin.  Since this bug is
five years old already and it doesn't look like it will ever be fixed,
all libsmbclient code in MPD is scheduled for removal.  For now, the
plugin is disabled by default so people are less likely to hit the
crash bug.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/991

Closes https://github.com/MusicPlayerDaemon/MPD/issues/981

Reduces header dependencies.

This got lost in commit 5d597a36 (v0.21.19), but it was never
noticed because the state_file_interval was way too short due to
commit 3413d1bf, fixed recently by commit 27cc7b35