pub fn register_child_hook(flag: Arc<AtomicBool>) {
    _ = signal_hook::flag::register(signal_hook::consts::signal::SIGCHLD, flag)
        .map_err(|err| tracing::error!("Cannot register SIGCHLD signal handler: {:?}", err));
}
Register the `SIGCHLD` signal handler. Once the signal is received,
the flag will be set true. User needs to manually clear the flag.
leftwm-core/src/utils/child_process.rs