Support setting region from secondary selection and vice versa

Support setting region from secondary selection and vice versa

* lisp/mouse.el (secondary-selection-exist-p): New function to
allow callers to tell existence of the secondary selection
in current buffer.
(secondary-selection-to-region): New function to set
beginning and end of the region from those of the secondary
selection.
(secondary-selection-from-region): New function to set
beginning and end of the secondary selection from those of
the region.  (Bug#27530)

* etc/NEWS: Mention the new functions.