[APT] Add checks for suspicious behaviour after installing .deb files

[dominichayesferen]

tl;dr: Add a check after installation/updating finishes to report to the user if suspicious activity has been detected.

This discussion will mainly consist of ideas for suspicious behaviour to check for.

[dominichayesferen]

Added cron jobs
If there are new cron jobs that do not belong to a package, and did not exist before the installation started, suggest suspicious activity to the user, pointing to the added files via a list of sorts.

[dominichayesferen]

Added systemd tasks
Same as cron jobs, but with systemd tasks as it's incredibly uncommon for postinsts to add systemd tasks in runtime, nevermind at all outside of the tasks they add by virtue of being contents of the .deb package itself.