Fix a horrible race condition -- various routines were storing the

Fix a horrible race condition -- various routines were storing the
most recently opened URL in self.openedurl of the URLopener instance.
This doesn't really work if multiple threads share the same opener
instance!

Fix: openedurl was actually simply the type prefix (e.g. "http:")
followed by the rest of the URL; since the rest of the URL is
available and the type is effectively determined by where you are in
the code, I can reconstruct the full URL easily, e.g. "http:" + url.