Add a new method `CommandExt::session_leader(&mut self, on: bool)` to create a new session (cf. `setsid(2)`) for the child process. This means that the child is the leader of a new process group. The parent process remains the child reaper of the new process.
This is not enough to create a daemon process. The *init* process should be the child reaper of a daemon. This can be achieved if the parent process exit. Moreover, a daemon should not have a controlling terminal. To acheive this, a session leader (the child) must spawn another process (the daemon) in the same session.
cc rust-lang/rfcs#941
cc #17176
6afb8f58d1