ndisc: Accept routes from on-link prefixes form ra

It is possible that an ra leads to two routes having the same prefix as well as the same prefix length. One of them, however, refers to the on-link prefix, and the other one to a route from the route information field. (Moreover, they might have different route preferences.) Hence, if both routes differ in the on-link property, both are added, and the route from the route information option receives a metric penalty. Fixed #1163 (closed).