Provide control port event+command for network connectivity info
For #11222 (moved), it would be useful to have a control port event and GETINFO command to learn Tor's current opinion about overall network connectivity, so we can try to differentiate censorship from general network outage.
One potential source of this network liveness info is via circuit_build_times_network_is_live() and circuit_build_times_network_close(). There may be other pieces of Tor that try to decide if the network is up too, though. I seem to recall athena working on some piece of code that needed to do this, but I can't remember the ticket.
Alternatively, there may be more accurate platform-specific mechanisms we could use instead, I bet. Most OSes have a notion of if it's interfaces are connected and sending/receiving packets recently. OS info may prove more useful for this use case than the CBT code is.