Fix `Pty::setFlowControlEnabled` to only set `IXON`, not `(IXON | IXOFF)`
Removed IXOFF flag and renamed _xonXoff to just _xon.
tldr: IXOFF is for physical ttys, not ptys, and every other terminal
emulator out there doesn't set IXOFF. It sticks out like a sore thumb
when running $ stty, and was probably incorrectly implemented due to
man 1 stty's crust. See bug 495276 for the full writeup.