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
Because you're using defer errors.Maskf, I think you should just return onAbort(err, ...).

And because you're using errors.NotFoundf, you should use defer errors.Contextf, instead of Maskf, otherwise the NotFoundError will be lost.