fn become_subreaper()Expand description
Makes the supervisor the parent of every orphan among its descendants.
Without ptrace the supervisor reads and writes tracee memory on its own
authority, which Yama’s default ptrace_scope of 1 grants only towards
descendants. A daemon that double-forks would be reparented to init and
lose that standing; as a child subreaper the supervisor adopts it
instead, and wait_root reaps it.