20230727: x11 forwarding on ma.sdf.org

 From <<https://unix.stackexchange.com/questions/12755/how-to-forward-x-over-ssh-to-run-graphics-applications-remotely>>:

- Created /etc/ssh/sshd_config

- Added "X11Forwarding yes"

- ssh -v -X kkwoo@ma.sdf.org

- PVT: "echo $DISPLAY" shows some non-blank result

- PVT: run xeyes

- Result: R plot is OK but R View(cars$disp) is slow

- Result: firefox loads up but is pretty unresponsive. Unable to enter a web address in the location bar