inheriting processes from other ttys

[Ron Natalie]

There is no good way to subsitute one terminal file descriptor for
another.  Some programs may be smart enough (like emacs) to switch
for you, but this is the exception rather than the rule.  The only
practical way of doing this currently in UNIX, is to keep a psuedo-
terminal of some sort as the tty file descriptor and switch that
pseudo-terminal to real terminal mapping around.  This is a generalization
of the System V xt/sxt system.

This provides you the attach/detach features that some of the nicer
DEC operating systems provided (TOPS-20 anyone?).  I have a user
mode version of this for 4BSD (actually anything with Berkeley like
pty/tty pairs) that I did as a proof of concept for a later in-kernel
implentation, but I've never gotten around to finishing the in-kernel
project.

-Ron