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.
