systemd: don't try to run as user manager when called without any arguments

systemd: don't try to run as user manager when called without any arguments

It's better for users if programs don't do "significant" things too easily, and
should be especially conservative when called without any arguments whatsoever.
So far systemd would would try to launch itself as a user manager and fail on
some cgroup permission stuff. systemd --user is run execlusively from user@.service
and there we call it with --user. Calls to the binary without any arguments as
non-pid1 are almost always a mistake.

https://github.com/systemd/systemd/issues/18419#issuecomment-779422571