1. We should make the `post-fs-data.sh` use itself a process group, otherwise it will receive signal from other process and make it exit accidentally.
2. And we should also switch child process' cgroup out of parent process, otherwise it would be kill by cgroup.
When executing module install scripts, KSU is set to "true"; when executing module post-fs-data scripts, KSU is set to "1". This may cause confusions so use the same value instead.