rdesktop patch to add limited RDPv6 + CredSSP

I have recently finished a project that adds support for limited aspects of RDPv6 to rdesktop. Specifically I have added support for SSL and SSL + CredSSP. These are the protocols that must be used when "High Security" or "Network Level Authentication" is enabled on the server side. The patch has been tested against Server 2008, 2008R2 and Windows 7 and both scenarios are working well.

A few notes:

We hope these are useful to the community and can help to jump-start RDPv6 support in the rdesktop trunk.

- David Joyner