zebra: static: update on ifindex changes

zebra: static: update on ifindex changes

Whenever an interface is created or deleted in the system, we need to
check whether we have static routes referencing that interface by name.
If so, we need to [un]install these routes.

This has the unfortunate side effect of making static routes with
non-existent interfaces disappear from "show ip route", but I think
that's acceptable (and I don't see a "good" fix for that).

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>