Resolve alter_context_auth

NTLMSSP negotiation alters instance variables in the Authentication
class. The auth_ntlm_3 method was being called as a class method
by Packet and failing to set nonexistent class variables.

Rewire auth_ntlm_3 as an instance method, call as such from Packet.

Current error: "Exploit failed: ArgumentError bad value for range."
The streak of informative stack traces may be over.