GSSAPI support on Windows, attempt 1

These patches add GSSAPI support on Windows, allow the connection string builder to get the user's Active Directory username in the proper case (and with optional realm), and close a security hole with regards to connection pooling and integrated security.

Guessing the user's Active Directory name requires an additional reference to to the System.DirectoryServices.AccountManagement library, but from what I can tell, Mono supports this library. If that's a deal breaker, the 23a931d commit can be omitted, but 26195b5 is critical.