- kernel ignores rtm_tos for IPv6 routes. While iproute2 accepts it,
  let libnm reject TOS attribute for routes as well.

- move the tos field from NMPlatformIPRoute to NMPlatformIP4Route.

- the tos field is part of the weak-id of an IPv4 route. Meaning,
  `ip route add` can add routes that only differ by their TOS.