If a filename on Windows starts with \\, it is converted to a URL
which starts with ////.  If this URL is passed to urlparse.urlparse
you get a path that starts with // (and an empty netloc).  If you pass
the result back to urlparse.urlunparse, you get a URL that starts with
//, which is parsed differently by urlparse.urlparse.  The fix is to
add the (empty) netloc with accompanying slashes if the path in
urlunparse starts with //.  Do this for all schemes that use a netloc.