Periodically check the integrity of installed binaries

[KellerFuchs]

I think we could/should run debsums as a cronjob (likely with ionice) so that we learn if, in the future, some installed packages get hit by FS corruption.

This is not meant as a HIDS (Host-based Intrusion Detection System), just as a rough FS consistency check (and it tends to catch annoying-to-debug issues).