SMB2: Separate RawNTLMSSP authentication from SMB2_sess_setup

We split the rawntlmssp authentication into negotiate and
authencate parts. We also clean up the code and add helpers.
Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>