Provide a way for privileged users to interact with systemd user instances

Description of problem:

With RHEL8.x, users can have their own systemd instance, but there is no way for the root user or privileged users (e.g. in "wheel" group) to manage the systemd user instances at all.

So far, the only way is to use "sudo+XDG_RUNTIME_DIR" which is not a good solution:

$ sudo -u user1 /bin/bash -c "export XDG_RUNTIME_DIR=/run/user/$(id -u user1); /usr/bin/systemctl --user list-units"

Version-Release number of selected component (if applicable):

systemd-239+

How reproducible:

N/A

Additional info:

See also:
https://github.com/systemd/systemd/issues/14580