s3-auth: Follow auth_ntlmssp and use auth4_context for Session Setup

s3-auth: Follow auth_ntlmssp and use auth4_context for Session Setup

This patch ensures consistency in behaviour between NTLMSSP and NTLM
session setup handlers.  By calling the same layer that auth_ntlmssp
calls, we can not only allow redirection of all authentication to the
AD DC, we ensure that map to guest and username map handling is
consistent, even in the file server alone.

Andrew Bartlett