Don't use IgnoreError() when there's an overload that does not try to
give us one.

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

Replaces GLib's GError.

Use the C++ API.

The song's URI must be UTF-8, not filesystem character set.

Allow the plugin to allocate the GetSong() return value.

Shortcut for map_directory_fs(db_get_root()).

When we have an absolute path that's not inside the music directory,
allow loading it anyway, if we're in "secure" mode (i.e. the client is
connected via UNIX socket).

Right now, a playlist with absolute pathnames can only add songs that
are in the same the directory of the playlist or under it.

If uri is an absolute pathname and base_uri is set,
playlist_check_translate_song() will check that base_uri is a prefix
of uri, excluding every other song in the music directory outside
base_uri.

I think in this case base_uri should be completely ignored (and made
NULL) and uri should just be checked against music root directory.

g_path_get_dirname() returns "." when there is no directory name in
the given path.  This patch adds a workaround for that.

Optionally allow all local files.

"Insecure" mode is used for printing playlists.

Free the temporary path string in apply_song_metadata().