Until now, we were leaving services "enabled"
(symlinks in /etc/systemd/system/multi-user.target.wants/).

We clean these up now. Broken symlinks may still exist in older
installations that enabled/disabled services. We're not taking care
to fix these up. It's just a cosmetic defect anyway.

Same reasoning as in 1cd251ed78

These validation tasks should only run when Prometheus is enabled.

Basic system stats, to show stuff the synapse metrics
can't show such as resource usage by bridges, etc

Seems to work fine as well.

This too has only been tested on debian amd64 so far