Have APT directly use the Tor SOCKS proxy (Closes: #8194).

At the end of the chroot_local-hooks stage, we replace the hook that used to
tell APT to use the Polipo HTTP proxy with another one that does (a more
elaborate version) s,http://,tor+http://, in APT sources.

Then, APT will use the "tor+http" method, that we install in
/usr/lib/apt/methods, and that is a simple torsocks wrapper for the
"http" method.
Move code for toggling tor+http to a dedicated, more generic program.
Adjust hook that includes syslinux in the ISO filesystem to APT being moved from HTTP to SOCKS.

Don't run apt-get update in the chroot again after reconfiguring APT sources
for runtime.

This is broken by the move to torsocks. Back when we used a HTTP proxy, the
HTTP_PROXY environment variable passed by the build environment was overriding
the APT proxy configured for runtime. This is not the case anymore, and now that
APT run tries to use both the tor+http shim and the HTTP proxy used in the build
environment, which is rejected by torsocks.

Besides, the build process works just fine without this "apt-get update" run,
so let's just drop it.
Add design documentation wrt. how we configure APT.
Revert "Workaround build failure in lb_source, after creating the ISO." (Closes: #5307)

This reverts commit 4218f2e47bf0ecc2bccaaed2140cc97ad6fc0ff4.

This is not needed anymore given the move to the Tor SOCKS proxy.
Fix punctuation in comment
Merge remote-tracking branch 'origin/feature/8194-APT-socks' into devel