BSD: Listen for Router -> Host messages via RTM_CHANGE
BSD: Listen for Router -> Host messages via RTM_CHANGE

If there is a lladdr in the gateway address then the node
has become reachable. If there is no lladdr in the gateway address
then the node is no longer a router.

Unreachable is sent via RTM_DELETE and we ignore the initial
RTM_ADD without a lladdr.

rsmarples/dhcpcd,rsmarples/dhcpcd,rsmarples/dhcpcd