Revert "Completely rewrite with python to use setpgrp instead of setsid"

This does not work well.
When the background process execute another
background process, the executed process is not killed.

This reverts commit c16d89987a7c49f6b8452ffd77444ed12b78021e.
Revert "Update README"

This reverts commit 9630fb97f4a9f7740810b68f6f86304e9323c748.