NTLM: ignore CURLOPT_FORBID_REUSE during NTLM HTTP auth

Problem: if CURLOPT_FORBID_REUSE is set, requests using NTLM failed
since NTLM requires multiple requests that re-use the same connection
for the authentication to work

Solution: Ignore the forbid reuse flag in case the NTLM authentication
handshake is in progress, according to the NTLM state flag.

Fixed known bug #77.