[Xcb] renaming the XCBConnSetup* mess

[Vincent Torri]

> As I've mentioned in bug #6757, Josh and I would like to rename the
> XCBConnSetup* types and functions. We think that replacing
> XCBConnSetupSuccessRepRootsIter with XCBSetupRootsIter, for example, is
> much clearer and easier.

As the function that retrieves the setup is XCBGetSetup, i'm for it. Or
add 'Conn' and 'Success' to XCBGetSetup :)

Vincent