.. and move the plugins to src/encoder/plugins/.

This file has been unmaintained for 5 years, and nowadays, MPD tries
very hard to stay compatible with older versions.

There will only be very few items, and that does not justify the bloat
of std::map.

Fixes linker failure.

Reduce bloat.

Reduce bloat.

For debugging.

Add missing server name prefix.

Don't use std::string, reduce bloat.

Not necessary because we control the LightSong's life cycle.

Zero means "unknown".