Add systemd preset

[RyanSquared]

https://www.freedesktop.org/software/systemd/man/systemd.preset.html

This helps stop issues such as units enabling themselves on-install or on-update.

[RyanSquared]

I think we should do this on a case by case basis for units that we want restarted. I dunno the original context of this. Maybe it was because of an nginx installation?

[daurnimator]

The problem was that we'd e.g. install nginx (so that users can use it as a user); but debian defaults to starting all daemons on installation.

You can write a systemd-preset so that installing doesn't automatically = starting.