Update kill -TERM fallback to check that systemd is PID 1
    
    Also only execute the fallback when we're upgrading the RPM package.
    
    Add a comment to indicate the actual bug in systemd v239 we're trying to
    fix with this fallback.
    
    Tested: Upgraded from v239 on a machine and confirmed that running
    `sudo systemctl status` was working as expected after the upgrade, rather
    than failing with "Access denied."