Network Cookie Jar: implement assignment operator.

This change is needed for implementing cookie persistence in Web+ using
the network kit backend.

The current implementation requires the user to unarchive the cookie
jar, then hand it over to the BUrlContext which will copy it to its own
field. This makes the code simpler, but maybe doing a complete copy
(with all the cookies) is an heavy operation and could be avoided.
Improve HTTP authentication support.

The authentication state is stored (in a hash map, using the domain+path
as a key) in the UrlContext class. It can then be reused for multiple
requests to the same place. We also lookup stored authentications for
parent directories and stop at the first we find.

Authentication state is not stored on disk (unlike cookies), and there
can only be one for each domain+path.