Adjust for HttpResult rename in Haiku Network Kit.

 * This also gets file: URLs working.
Comment out dead code.
Authentication support.

Most code comes from curl implementation, but we don't do everything
the same: we handle authentication like a redirection, by creating a
new request, instead of reusing the same one. This allows working
asynchronously.
Cookie Jar: do not leak the previous cookie jar

... when setting a new one.