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
To be on the conservative side, I'd add `.Select(bson.D{{"_id", 1}, {"isdisabled", 1}}).` after Find().