Support client authentication using Winbind

Support client authentication using Winbind

Based on a patch by David Woodhouse <David.Woodhouse@intel.com>
Original commit message:
    We need to screw around with the flags a little, since winbind doesn't
    really get it right. Thankfully, it doesn't support MIC and it does at
    least generally do the right thing (w.r.t. session negotiation and OEM
    vs. Unicode) so it's sufficient just to screw with the flags.

    Tested with Negotiate authentication to squid, and NTLM in datagram
    mode with pidgin-sipe. Also with Firefox, Chrome and a fixed libcurl.