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. :)

I'd like to thank @joeyh for pointing out the root of this problem - we have some light documentation in this script about why we do these things, but definitely no documentation actually inside the image itself for what the heck these things are for and the rationale for them, so I've added comments to hopefully help explain a little bit of the "why" so users of the image aren't left scratching their heads at how odd we are. :)

Also worth noting, we used to symlink "/sbin/initctl" to "/bin/true", but that doesn't really provide any visibility into the "why" of the change, so it is instead now a short script copied from the "policy-rc.d" script and changed to `exit 0` which gives us that comment that hopefully helps explain why it's there.

Fixes #5522