New user variable `exchange-point-and-mark-highlight-region`

New user variable `exchange-point-and-mark-highlight-region`

When set to nil, this modifies `exchange-point-and-mark' so that it doesn't
activate mark when it is not already active.

* etc/NEWS: Introduce new variable
exchange-point-and-mark-highlight-region.

* lisp/simple.el
(exchange-point-and-mark-highlight-region): New.
(exchange-point-and-mark): Use it.

(cherry picked from commit ef79c8cf3d63f510107536901925c05e37740275)