Enable or disable ^S/^Q flow control for output to TERMINAL.
If FLOW is non-nil, flow control is enabled and you cannot use C-s or
C-q in key sequences.
This setting only has an effect on tty terminals and only when
Emacs reads input in CBREAK mode; see `set-input-interrupt-mode'.
See also `current-input-mode'.
(fn FLOW &optional TERMINAL)