[Mono.Security]: Change NTLM domain policy.

There has been a bug report where a Microsoft Server 2008 RC2 does
not accept the domain name that it sent back in the Type2Message's
TargetInfo / Domain Name field.

Therefor, we're now using the domain name from the `NetworkCredential' -
this is also what's initialy being sent to the server in the
`Type1Message`.