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

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

Use the same chunk size for all binary commands.

Move code from read_stream_art().

Why would anybody want to keep the silence inserted by the codec?
Other plugins/codecs (such as Vorbis) have this hard-coded as well.

SID files are generally collections of tunes, so a SID name field makes
sense as an MPD album. The SID tune information list (STIL) has name
and title fields for individual tunes, when such are known, but MPD is
currently not using the STIL.