Add NTLM SIGN flag for SMB Server

[jborean93]

Adds the NTLMSSP_NEGOTIATE_SIGN flag to the NTLM CHALLENGE message returned by the SMB server. This is needeed for clients that generate a SPNEGO mechListMIC which require signing to be enabled on the NTLM context.

Some background information, a client that sends an NTLM token wrapped in a SPNEGO payload can set the mechListMIC but the MS-NLMP docs are unclear what the signature behaviour is without the NTLMSSP_NEGOTIATE_SIGN or NTLMSSP_NEGOTIATE_ALWAYS_SIGN flags. I've interpreted it as signing isn't supported and thus the mechListMIC field cannot be computed. What should happen is if the client itself specified the NTLMSSP_NEGOTIATE_SIGN flag then the challenge should also respond with it which this change does. This is backed up by the protocol docs for CHALLENGE_MESSAGE

If the client has set the NTLMSSP_NEGOTIATE_SIGN in the NEGOTIATE_MESSAGE the
Server MUST return it.