This eliminates some complexity from class CurlRequest.

Hooray 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

Found with modernize-avoid-bind
Signed-off-by: Rosen Penev <rosenp@gmail.com>

To be reused by the FFmpeg filter plugin.

This compatibility macro has been removed from FFmpeg long ago.

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

Reduces header dependencies.

snprintf() is available on mingw, and the libnfs kludge broke the
build with mingw, because sprintf_s() was now both an inline function
and a "dllimport" function (because the macro renamed the inline
function snprintf() to sprintf_s() in mingw's stdio.h).

Keep the logging library out of this low-level libcurl wrapper.

We need at least GCC 8 anyway.

Similar to commits 1686f4e8 and
30a5dd26

Instead of using this as a base class implementing a virtual method,
the new class IdleEvent can be used as a variable, decoupling
IdleMonitor's internal state from the derived class.

This is similar to commit 30a5dd26
which refactored TimeoutMonitor to TimerEvent.

GCC5 cannot build mpd.
Signed-off-by: Rosen Penev <rosenp@gmail.com>