For link local addresses the XDMCP server would need to either know the
interface thru a scope identifier or try all available interfaces. If
they don't this address will fail in which case the XDMCP server could
still try the other addresses passed - however some only try the first
address and then give up.
Even if this seems to be the wrong place to fix this it seems to be
easier than fixing all display servers.
[ajax: Cleaned up commit message]
Reviewed-by: Adam Jackson <ajax@redhat.com>