s4:ntlmssp Use common code for ntlmssp_sign.c

s4:ntlmssp Use common code for ntlmssp_sign.c

The common code does not have a mem_ctx on ntlmssp_check_packet() and
ntlmssp_unseal_packet().

We do however need some internal working of the code exposed, so some
structures are moved to ntlmssp_sign.h

Andrew Bartlett