This moves tracking of connectivity to NMDevice and makes the NMManager
negotiate the best of known connectivity states of devices. The NMConnectivity
singleton handles its own configuration and scheduling of the permission
checks, but otherwise greatly simplifies it.

This will be useful to determine correct metrics for multiple default routes
depending on actual internet connectivity.

The per-device connection checks is not yet exposed on the D-Bus, since they
probably should be per-address-family as well.