Hardcoded workstation name

[DanTheMan-7]

As for now the workstation name for the ntlm negotiation is hard coded in authenticate_message.go to workstation := toUnicode("go-ntlmssp"). It is not possible to set it manually. This can lead to problems with logs of intrusion detection systems, since they can log the client as the workstation name, what can lead to confusion. Therefore, it would be desirable to have the possibility to set the workstation name by oneself.