[GH-487] Moving init script to start-stop-daemon

This is a shortest and final version of https://github.com/boot2docker/boot2docker/pull/489 to not add 18 commits for one file :)
@SvenDowideit I added a little comment thru lines 15-16.

In fact :
- Due to some golang constraints, docker doesn't background itself (see https://github.com/docker/docker/issues/2758). When you launch it with the "-d" option, it is detached alone but cannot be manageable without another tool like "home-made script", start-stop-dameon, etc.
- So the "contrib" init script in the main docker project try to use start-stop-daemon, and ran into some race-conditions issues like https://github.com/docker/docker/issues/5359. 
- The idea they use in the "Debian" init script, is to allocate a specific PID to the parent process which role is to background the docker process. This "SSD_PID" is used to manage the parent process, forked from start-stop-daemon (see the doc here for reference - http://www.delafond.org/traducmanfr/deb/man8/start-stop-daemon.8.html).

See https://github.com/docker/docker/issues/2758 for details.

cc @tianon : your advise on that (it seems you got a foot in these thing in Docker :) ) ?