Hooray C++17!

This is obsolete since C++17.

This template was never instantiated, so the problem never occurred.

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