Introspection socket for privileged containers

Please comment / criticise this PR.

It add a socket in `/.dockersock` for privileged container. On this socket, the docker rest api is running, so it can be used for introspection.

Each privileged container has it's own socket, it'll be useful in the future.

As the socket is in the initlayer, non-privileged containers also have a `/.docketsock` file, with rights set to `0000`

It  doesn't work in tests, I get "bind: invalid argument", if @jpetazzo you have any idea, let me know.