Make upstream systemd-sysext reload the units by default

[pothos]

Make upstream systemd-sysext reload the units by default: systemd-sysext refresh should also issue a daemon-reload as done by systemctl enable X, this allows us to drop this line https://github.com/flatcar/init/blob/fbb1ec8e9d95598d65e4eac408dbc5f75b9b6138/systemd/system/ensure-sysext.service#L14

Since this will start units through Upholds= as upstream-supported way, document that mechanism in the systemd-sysext manpage.
@tormath1 mentioned that we should also look for any downsides compared to Wants=.
Edit: Filed systemd/systemd#26896 for that.

[tormath1]

Upstream PR is merged.

[tormath1]

As the issue concern the upstream feature, I guess we can close this one?

[pothos]

Yes, thanks a lot!
Do we want to backport this when we update to 254?

[tormath1]

@pothos patch can be applied correctly against v254-stable - there is a trivial conflict on the test/test-functions but it can be cleanly solved by backporting this commit too: systemd/systemd@b856f1d.
So yes, we can backport this during the update to 254 (and merge this: flatcar/init#96)