pub fn fixup_systemd_env()Expand description
When the proxy restarts itself, it sets the child’s LISTEN_PID env to a special value so that the child can replace it with the real child PID. Doing this is easier than reimplementing rust’s process spawn code just so we can call execvpe to replace the environment in the forked process.
This is usually called by RestartConfig::try_into_restart_task but this function is available
if it needs to be done at an earlier or more convenient time, such as the top of fn main().