Add how to prevent systemd from killing apt/screen

Even though I don't know why this happened to me in this situation, the way to run screen suggested in this change prevents multiple situations where systemd will, as it is supposed to, kill screen and apt.

One of those is if logind.conf has KillUserProcesses=yes, though that is currently not the default in Debian.

One complexity here is that even if you use sudo to become root and then run screen, it will still be inside the original users systemd session scope. The suggested change will instead run screen in the system scope instead of the session scope of the original user or root.