s3-ntlmssp split auth_ntlmssp_client_start() into two parts

s3-ntlmssp split auth_ntlmssp_client_start() into two parts

This will allow it to be a wrapper around a gensec module, which
requires that they options be set on a context, but before the
mechanism is started.

This also simplfies the callers, by moving the lp_*() calls
into one place.

Andrew Bartlett