TANDEM mode bugs in 4.3bsd

[Guy Harris]

> 	A historical interest note: on vanilla v6 UNIX, XOFF was a toggle;
> send one XOFF to stop output, another to restart it.  Given that v6 didn't
> come with a pager program, the small amount of time saved by not moving
> your fingers on the keyboard was important at 9600 baud.

Vanilla V6 didn't have *any* support of XOFF, whether with XON or not.  Then
again, I don't think many people ran truly vanilla V6 (as in "read it in off
the tape and don't go near the source").