WIP: [klibc] ipconfig: Add support for RA-DNS alongside SLAAC

Add support for DNS options in Router Advertisements (RFC 8106).  As
with SLAAC, we wait for the kernel to receive this information and
notify us through rtnetlink.

These notifications are separate from new address notification, and
there's unfortunately no way to know that an RA was received without
DNS options.

TODO: add sensible timeout
TODO: should RA-DNS be optional?