systemd-machined: unprivileged users can terminate arbitrary processes

When systemd-machined >= v259 (or v258 with a custom `polkit` policy that allows `register-machine` access) is running on a desktop system, an unprivileged user logged in a desktop graphical session can kill arbitrary processes, even privileged ones.

- versions older than v259 are not affected, unless unprivileged access is granted for the `register-machine` polkit action via a local, custom policy config file
- versions older than v258 are not affected
- unrelated to the systemd service manager (pid 1 or user session managers)
- systemd-machined is not typically installed by default, and is typically in an optional, separate package (e.g.: systemd-container)
- terminal-only or remote sessions (e.g.: ssh) are not affected