Wait for the first child to fork before exiting

[bartelsielski]

Without the waitpid() it is possible that the setsid() has not completed yet when the main process terminates, potentially causing a race condition:

If setsid() has not completed and the controlling terminal is closed, the session leader will be sent a SIGHUP signal. By default this terminates the process and then all processes in the session group will also receive a SIGHUP.