Remove the C list_head library and use type-safe C++ instead.

A SimpleDatabase instance can now "mount" other Database instances at
certain locations.  This is used to use a new SimpleDatabase instance
for each storage mount (issued with the "mount" protocol command).
Each such instance has its own database file, stored in the directory
that is specified with the "cache_directory" option.

Helps avoid unnecessary archive scans during database update on a
fresh MPD process.

The Song class is only used for database songs now.  A Song without a
Directory is not possible anymore.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

Not necessary anymore for playlist_metadata_load().

Replaces GLib's g_str_has_prefix().

Replaces GLib's GError.

Wrap songvec_find() and other songvec methods.

Random access is not needed, and a linked list is easier to manage: we
don't need to (re-)allocate the pointer array.