Update contrib/mkimage/debootstrap to add "sqeeze-lts" to "oldstable" as well

Note that the archive does _not_ have a symlink from oldstable-lts to squeeze-lts, hence the change from using "${suite}" to not using it in here and just using the string "squeeze-lts" directly regardless of whether we're building a "squeeze" image or an "oldstable" image.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
understanding-docker: Use consistent term

Term 'service' was used together with 'daemon'. It would be better to use the term 'daemon' solely because most part of documentation are using the term.
Merge pull request #6899 from sjp38/patch-1

understanding-docker: Use consistent term
Merge pull request #6897 from tianon/moar-squeeze-lts

Update contrib/mkimage/debootstrap to add "sqeeze-lts" to "oldstable" as well