Proper way to detach from control terminal?

[John Young]

A additional comment regarding process groups on SVR3.1 and up.
A change to setpgrp() makes it only effective the first time.
This means that after a tty link first goes down (and you get
your first SIGHUP) there is no way to get further SIGHUP indications
(by the same process).  That's progress for you!  If anyone knows
otherwise please inform me.