Currently we call
carrier_changed() every time the carrier goes up again and the device
is activating, to kick a restart of DHCP.
Since we process link events in a idle handler, it can happen that the
handler is called only once for different events; in particular
device_link_changed() might be called once for a link-down/link-up
carrier_changed() is "level-triggered" - it cares only about the
current carrier state.
instead be "edge-triggered" - invoked every time the link goes from
down to up. We have a mechanism for that in