setsid() creates a new session if the calling process is not a process
group leader. The calling process is the leader of the new session, the
process group leader of the new process group, and has no controlling tty. The
process group ID and session ID of the calling process are set to the PID of
the calling process. The calling process will be the only process in this new
process group and in this new session.
On error, -1 will be returned. The only error which can happen is EPERM. It is
returned when the process group ID of any process equals the PID of the
calling process. Thus, in particular, setsid fails if the calling
process is already a process group leader.