NTLM: handle auth for only a single request

Currently when the server responds with 401 on NTLM authenticated
connection (re-used) we consider it to have failed.  However this is
legitimate and may happen when for example IIS is set configured to
'authPersistSingleRequest' or when the request goes thru a proxy (with
'via' header).

Implemented by imploying an additional state once a connection is
re-used to indicate that if we receive 401 we need to restart
authentication.

Closes #363