WIP: Machine.WatchInterfaces NotifyWatcher

state:WatchInterfaces

This is a NotifyWatcher that monitors the machine's network interfaces, and reports the Ids of network interfaces that are modified.

This should catch Added, Removed, Enabled, Disabled
s/%v: %v/%q on machine %q: %v/ and use ni.InterfaceName(), ni.MachineId().

Using %v here gives too much internal information.