Add jumpers to PCB design to switch flow control from RTS/CTS to DTR/DSR

[ssshake]

Some computers use DTR/DSR for flow control. We can't control this in software because the max3232 chip only had 4 physical connections to the serial port. So I think adding two jumped to the PCB would allow a user to switch between the common flow control pins to the less popular pins.

I think NEXT computers were like this.

https://tldp.org/HOWTO/Text-Terminal-HOWTO-11.html