httpclient: support authentication

Store the last-seen credential challenges (eg, all the
'WWW-Authenticate' headers in a response message).  Given some
credentials, find the best (first) challenge whose mechanism supports
these credentials.  (eg, 'Basic' supports username/password credentials,
'Negotiate' supports default credentials).

Set up an authentication context for this mechanism and these
credentials.  Continue exchanging challenge/responses until we're
authenticated.

Author: ethomson