Add docker upstart pre-stop and post-start events to prevent race conditions

An attempt to fix issue #6647
Since you're not waiting for the socket to exist here, how do you know Docker is actually up at this point?  Won't this have the same racy behavior as just waiting for the "docker" event, or is there some Upstart magic here I'm not familiar with? :)  (My knowledge of the deeper, darker Upstart magic is admittedly lax.)