Signed-off-by: Rosen Penev <rosenp@gmail.com>

Similar to commit 4e2a551f but for
decoder plugins.  This is tailored for the FFmpeg decoder plugin which
implements some protocols (e.g. RTSP) as demuxer plugin.

Bug #915 is about an I/O exception thrown where none was allowed,
leading to crash via std::terminate().  However, instead of catching
and logging the error inside the decoder plugin, it should be able to
propagate the I/O error to the MPD core, so MPD can avoid trying other
decoder plugins.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/915

Add a `constexpr` constructor and several `constexpr` methods to
construct a DecoderPlugin at compile time, in a way which allows
adding new methods later without having to edit each plugin.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Speed up container_scan() again, by eliminating the need to call
scan_file() for each item.

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

The old struct config_param remains only for top-level string options.