Do not choose a route whose next hop cannot be reached (RFC 4271 §9.1.2.1)

Where routes go into the kernel, a route the kernel could not be given
is no longer taken into the table: its next hop is no neighbor on any
interface, or its prefix is a network the machine is itself on. Such a
route used to be able to win selection and then leave its prefix without
a route in the kernel, though another peer had one that worked.

It is held aside the way an RPKI-invalid route is, and by the same
means: judged on arrival against the interfaces as the kernel last
described them, looked for in the table when they change, and taken
from what is held when its next hop can be reached. The fib publishes
the interfaces for this; the next hop is resolved once for an UPDATE
and not for each of its routes.

The lab has a phase, nexthops: FRR's better route with a next hop that
is nowhere is held and BIRD's is installed, and they change places as
an address comes and goes.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/RFCs.md

@@ -33,7 +33,8 @@ The base protocol specification.
 | §6.2 | Bad BGP Identifier check (peer using our identifier, or 0.0.0.0) | ✅ |
 | §9 | Decision process / best-path selection | ✅ |
 | §9.1.2 | Best routes installed for forwarding (Linux kernel, over netlink), where the next hop is directly connected | ✅ |
-| §9.1.2.1 | Routes with an unresolvable next hop excluded from selection; next hops resolved through other routes | ❌ (such a route can be chosen, and is then not installed) |
+| §9.1.2.1 | Routes with an unresolvable next hop excluded from selection | ✅ where routes are installed in the kernel: such a route is held out of the table until its next hop can be reached |
+| §9.1.2.1 | Next hops resolved through other routes | ❌ (only next hops that are neighbors count as reachable) |
 | §9 | Adj-RIB-In (per-peer table of received routes) | ✅ |
 | §9 | Locally originated routes, advertised to all established peers | ✅ |
 | §9 | Loc-RIB / Adj-RIB-Out for routes learned from peers (re-advertisement) | ❌ |