Prevent $DISPLAY from being overridden in SSH (for X11 forwarding)

[InnovativeInventor]

When trying to ssh into a qubes vm, /etc/profile.d/qubes-gui.sh will improperly set $DISPLAY to be :0. When X11 forwarding is used, $DISPLAY is automatically set to ensure that the relevant windows get forwarded to the ssh client. However, since qubes-gui.sh is called after a successful login, the correct value for $DISPLAY will be overridden.

This commit fixes the issue by adding a test in qubes-gui.sh to check if we are currently in an ssh session (by checking if $SSH_CLIENT or $SSH_TTY are set).

Note: I do not know .csh scripts and have not tested the proposed change to qubes-gui.csh, yet.