Similar to the `user` option, the added `group` option sets the group of
the executing process. If not `null`, it also sets `DynamicUser=false`.
In case `user` is set to `null` (the default), systemd would run the
service as root implicitly. As this is dangerous and most certainly not
what users want, we force them to set `user = "root"` explicitly if
that's really their intention. That's achieved through an assertion.
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