Fix docker init script

This is the PR related to https://github.com/boot2docker/boot2docker/issues/487
It seems thats not enough. maybe we should take more time to understand the behaviour i encounter.
The problem in my case is that the original Docker daemon never stop, it never releases the sockets, resulting in a broken boot2docker.

I tought too the "kill -9" option is very agressive and uncommon. I'm not sure where to go : using start_stop_daemon like in the debian/redhat-sysv systems ?