Addition of rc-loss duration to the mavlink warning messages

See above. The first commit (5c77fc0) simply adds the time (since bootup) when the rc-loss/regain happened. The second commit adds the duration of that rc-loss. This is implemented by adding a variable to the vehicle_status_s-topic, but could also be done through a local variable if that is preferred. This code has been bench-tested.
This has already 45 characters length, with the 50 character limit of the total length it might make sense to remove the (at t= xxx statement) or otherwise shorten it so that it can sustain a long, but still reasonable flight time.