Allow selecting region with mouse to move point to beginning

Allow selecting region with mouse to move point to beginning

* etc/NEWS: Mention new user option
`mouse-select-region-move-to-beginning'.

* doc/emacs/frames.texi (Mouse Commands): Add cross-reference
to the following.
(Word and Line Mouse): Describe how double-clicking mouse-1 to
activate region and `mouse-select-region-move-to-beginning'
affect point.

* lisp/mouse.el (mouse-select-region-move-to-beginning): New defcustom.
(mouse-set-point): Use it.  (Bug#23478)