s4:ntlmssp Merge more aspects of the source3/ NTLMSSP layer

s4:ntlmssp Merge more aspects of the source3/ NTLMSSP layer

This changes the talloc treatment of the session keys to avoid
memory duplication - the session key has always been allocated
onto the ntlmssp_context by the auth subsystem callback.

The remainder of the changes are cosmetics, such as avoiding
using lm_session_key as a pointer (and avoiding then doing an
if statement on something that is always true).

Andrew Bartlett