Grouping in the "list" command was completely broken from the start,
unlike "count group".  I have no idea what I have been thinking when I
wrote commit ae178c77, but it didn't
make any sense.

This commit is a rewrite of the feature.

For clients to be able to detect this feature, this commit also
increments the protocol version.

Remove them as parameters from `db_selection_print()`.

Closes #172

Implement the second part of https://bugs.musicpd.org/view.php?id=3990

Lists files and directories.  Supports storage plugins.

Replaces GLib's GError.

The "bool" type is built-in.

Let the database plugin do the match.

Merge locate_song_search() and locate_song_match().