Simplify the plugin by using a trivial relative URI; later,
playlist_check_translate_song() will set the correct full URI.

Fixes -Wreturn-std-move (clang 8).

More for https://github.com/MusicPlayerDaemon/MPD/issues/42

Prepare for using other crypto libraries, e.g. FFmpeg's libavutil.

Use the "comments" attribute instead of relying on the nullptr
terminator.

Prepare for parsing large binary comments (such as
METADATA_BLOCK_PICTURE).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/42