(cua--preserve-mark-commands): New defvar.

(cua--preserve-mark-commands): New defvar.
Init to beginning-of-buffer and end-of-buffer.
(cua--undo-push-mark): New defvar.
(cua--pre-command-handler): Set inhibit-mark-movement if mark is
already active and command is in cua--preserve-mark-commands.
Also fix check for shift modifier on non-window systems.
(cua--post-command-handler): Clear inhibit-mark-movement if set.