don't reorder unknown interface types

don't reorder unknown interface types

Giving unknown interfaces an order-id of 0 caused them to
always be on top of the interfaces file. This is often
undesired. Instead we now only take type-ordering into
account when both interfaces which are being compared have a
known type, and otherwise only use the priority attribute.
This should result in a more stable modification of
interfaces.

Fixes #747