dirmngr: Add code to support the negotiation auth method.

* dirmngr/http.c (enum auth_negotiate_states): New.
(struct proxy_info_s): Add new fields.
(release_proxy_info): Free Windows stuff.
(proxy_get_token): New. Implemented only for Windows for now.
(run_proxy_connect): Add support for auth method Negotiation.
(store_header): Keep some header lines separate.

The code does something but I have not yet been able to test it due
to problems setting up Squid with AD authentication. As of now it
will respond with a failure but that should not be worse than not to
implement Negotiation.

Supporting Negotiation using GSS for Unix should eventually also be
done.

• GnuPG-bug-id: T6719