Property-based tests hitting the mgd static routing API produced some routes with a nexthop of 127.0.0.1; @rcgoodfellow pointed out that we ought to be rejecting this in both maghemite and omicron.

We've recently added some IpAddr-wrapping types in omicron that do this kind of validation; e.g., RouterIpAddr, which rejects localhost, multicast, ipv4-mapped-ipv6, etc. addresses. If we can come up with reasonable names, we could lift those types up to oxnet and reuse them across both omicron and maghemite.