Prepend the playlist's base URI to relative song URIs. Look up songs
in the database (if the URI refers to a local song file). Merge
existing database metadata with metadata from the playlist plugin.
After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.
This patch allows the client to load a playlist file from the playlist
directory with a plugin. This can be used with the "load" command,
but the client has to pass the file name including the suffix. We
will probably use the music directory in the future, to support
playlist files inside the music directory.
