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().

Use it in handle_lsinfo(), and eliminate some duplicate code.

Add nonnull attributes and fix all callers.

Use this struct for db_walk().

Allow calling findAddIn() without a client object.

Allocate a player_control object where needed, and pass it around.
Each "client" object is associated with a "player_control" instance.

This prepares multi-player support.