For routes and the default-route from NDisc, set the router preference
RTA_PREF.

Also, previously, we would only configure one IPv6 default-route. That by itself
was not really a problem, as long as NetworkManager would always make sure that
it configured the route to the ~best~ router.

Actually, NM should have done that already. It keeps the list of gateways
sorted, and prefers them according to their preference. But maybe
it didn't, so we have bug rh#1445417 (??).

Change that by configuring a default-route for all gateways, with
appropriate router prefrence. In case, kernel doesn't support RTA_PREF
yet, only configure all routes that share the same maxiumum preference.

https://bugzilla.redhat.com/show_bug.cgi?id=1445417