BUG/MINOR: only mark connections private if NTLM is detected

BUG/MINOR: only mark connections private if NTLM is detected

Instead of marking all connections that see a 401/407 response private
(for connection reuse), this patch detects a RFC4559/NTLM authentication
scheme and restricts the private setting to those connections.

This is so we can reuse connections with 401/407 responses with
deterministic load balancing algorithms later (which requires another fix).

This fixes the problem reported here by Elliot Barlas :

  https://discourse.haproxy.org/t/unable-to-configure-load-balancing-per-request-over-persistent-connection/3144

Should be backported to 1.8.