Bug#718225: live-build should authenticate files it downloads

[jnqnfe <jnqnfe@gmail.com>]

In fact I'm building a rebase on 5.x now, at least so I have a 4.x and
5.x branch to rework #773775 on top of (some of which conflicts with this).

The problem is though, and this is why I built it directly for 4.x,
there are conflicts with at least cdebootstrap removal and sparc removal
in 5.x. So either you've got to rebase the 4.x patch onto 5.x to get it
in 5.x and resolve the conflicts like I'm doing now, or if you cherry
pick this from 5.x, you're going to have to either have to have a
modified 4.x specific veriosn like I've just built, or otherwise you're
forced to also cherry pick the cdebootstrap and sparc removal commits
also, and I'm not sure that you'd want to push those to 4.x, though this
would be the easiest solution, considering that I doubt you'd want to
rebase 5.x onto this big solution of mine, considering the 5.x git
branch is already public.