GSSAPI support on Windows, attempt 2

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. Also adds a missing descriptor for ApplicationName in the connection string builder.

Guessing the user's Active Directory name requires an additional reference to to the System.DirectoryServices assembly, but from what I can tell, Mono supports this library. If that's a deal breaker, the 1f99be4 commit can be omitted, but you'll want the bug fixes in e90e9b5 and 3204d59.