s3:ntlmssp Split the NTLMSSP server into before and after authentication

s3:ntlmssp Split the NTLMSSP server into before and after authentication

This allows for a future where the auth subsystem is async, and the
session key generation needs to happen in a callback.

This code is originally reworked into this style by metze for the
source4/ implementation.

The other change here is to introduce an 'out_mem_ctx', which makes
the API match that used in source4.

Andrew Bartlett

Signed-off-by: Günther Deschner <gd@samba.org>