Add anonymous authentication support

[zeroSteiner]

These changes adjust some things to allow the NTLM session to authenticate anonymously (without a username and password).

For both NTLMv1 an NTLMv2, the docs call out that when the username and password are blank strings, the challenge fields should be special values.

If (User is set to "" && Passwd is set to "")
-- Special case for anonymous authentication
Set NtChallengeResponseLen to 0
Set NtChallengeResponseMaxLen to 0
Set NtChallengeResponseBufferOffset to 0
Set LmChallengeResponse to Z(1)
Else

This also adjusts the Message class to pad security buffer fields to a two byte boundary when the UNICODE flag is set. Per MS-NLMP Section 2.2.1.3 the user, domain and workstation fields must have offsets that are multiples of 2. A single null byte is used as padding when necessary. When the UNICODE flag is not set, no padding takes place. The padding is necessary because the code path executed when using anonymous authentication sets the lm_response field to a single null byte and shifts all fields after it.

The last change adjusts how the character encoding is determined. Per MS_NLMP Section 2.2.2.5, the UNICODE flag should take priority regardless of the value of the OEM flag. That is to say that when both are set, the "... choice of character set encoding MUST be Unicode".

Lastly some specs that exercise the updated code paths were added.

This was tested in combination with RubySMB and the anonymous_auth example.

[adfoster-r7]

Looks good to me 👍

From a quick look at the recent pull requests, I think @mwrock might still be active to help with publishing this 🤞

[adfoster-r7]

Let us know if there's anything we can do to help out @mwrock 🎉

[pcai]

thanks!