Supporting multipath on Gatekeeper servers

We should be able to get the relevant information to implement ECMP through the `RTA_MULTIPATH` attribute of the rtnetlink library. There's already a stub for handling this functionality in `cps/kni.c`.

We need to figure out how to use this information to implement ECMP, and do it only when running Grantor.

This feature should remove the relevant `XXX` in `cps/kni.c`.