modemmanager: corner cases where netifd interface gets out of sync w.r.t. the modem state in MM

There are two main situations that would lead to an interface with protocol "modemmanager" managed in netfid getting out of sync with respect to the associated modem object state in ModemManager:

• If netifd crashes or gets killed there won't be a clean ifdown procedure done for each interface. In this case, an interface that was reportedly "up" in netifd before the crash will end up reported "down" once netifd restarts, while the state of the modem in ModemManager doesn't change from "connected".

• If ModemManager crashes or gets killed, there won't be any notification to netifd that the modem got disconnected, so the state of the modem in ModemManager would not be "connected" any more, while the network interface status in netifd would still be "up".

We should somehow bind the state of both processes, so that abrupt crashes of any of them don't make them get out of sync completely.

Start by tracing how netifd's modemmanager protocol reports interface state and how ModemManager reports modem connection state across process restarts. Define handling for abrupt netifd and ModemManager termination, then verify that both states converge after either process is restarted.