Handle IPvX only interfaces with IPvY sessions more gracefully.

Handle IPvX only interfaces with IPvY sessions more gracefully.

In up_get_nexthop() check that the local_vX_addr is actually valid
before using it. In the UPDATE generation functions check that the
nexthop is valid before adding it and fail hard if it does not exist.
You can't announce an IPv4 prefix/gateway over an IPv6 only link.
OK henning@ sthen@