systemd: rewatch pids under cgroup v1 when sigchld of processes more than main pid...

systemd: rewatch pids under cgroup v1 when sigchld of processes more than main pid and control pid is captured

If `Delegate` is configured in service, cgroup agent will never send out
any datagram as .control subcgroup is generated. Thus systemd will watch
all processes on the cgroup hierarchy for SIGCHLD to deal with unreliable
cgroup notifications.

In this way, systemd should rewatch all processes when any SIGCHLD is
captured, more than the control pid or main pid.