n_tty_receive_char_flow_ctrl
if (I_IXON(tty) && n_tty_receive_char_flow_ctrl(tty, c, lookahead_done))
if (!n_tty_receive_char_flow_ctrl(tty, c, lookahead_done) &&
n_tty_receive_char_flow_ctrl(tty, *cp, false);