rpcrt4: Use SecInvalidateHandle and SecIsValidHandle for handling RpcConnection::ctx instead of setting/comparing the field to 0.

RPCRT4_CreateConnection fails by checking the return value from the
function and returning in this case.

(spotted by Peter Oberndorfer).

msxml3: Support both v 2.x and 3.0 version independent clsids as well as the v 3.0 version dependent clsid.

msxml3: Add a guard around DOMNodeType just like in the PSDK, so that we can include version 2 and version 3 msxml headers simultaneously.