Revert "units: ignore systemd-sysctl on shutdown"

This reverts commit faeffa73a81ab5b59acfadeb571431fb0e42af70.

There isn't really much point in dropping the Conflicts= since shutting
down this service is basically free as it doesn't have anything running.

Also, the patch was incomplete, because shutdown.target was still listed
in Before=.
unit: never retroactively start requisites

Requesites are not supposed to be auto-started afterall, they are just
checks, so don't try to be smarter here than appropriate.

Based on a patch from Michal Schmidt.
update TODO