Add a ton of simple improvements to contrib/mkimage/debootstrap, especially comments in the extra files put inside the image

This also adds support for squeeze-lts. :)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
be consistent in capitalization of Docker

Docker-DCO-1.1-Signed-off-by: Jonathan Boulle <jonathanboulle@gmail.com> (github: jonboulle)
Fatal() doesn't take a format string like Fatalf()

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)
Merge pull request #6572 from tianon/better-debootstrap-self-documentation

Add a ton of simple improvements to contrib/mkimage/debootstrap, especially comments in the extra files put inside the image
Merge pull request #6585 from kzys/runconfig-fatalf

Fatal() doesn't take a format string like Fatalf()
Merge pull request #6532 from jonboulle/fix_capitals

be consistent in capitalization of Docker