This allows a user to set a non-meta modifier for their terminal
should his/her terminal support it. See bug#35058 for background on
this change.
* lisp/emulation/cua-base.el (cua-rectangle-terminal-modifier-key):
New defcustom.
* lisp/emulation/cua-base.el (cua--shift-control-x-prefix): Use new
defcustom.