Let all active regions set the primary selection.

Let all active regions set the primary selection.
This includes both temporarily active regions (mouse drag and
shift-select) and those made with C-SPC and cursor motion.

* lisp/cus-start.el: Change defcustom for select-active-regions.

* lisp/simple.el (deactivate-mark): If select-active-regions is `only',
only set selection for temporarily active regions.

* src/insdel.c (prepare_to_modify_buffer): Handle `only' value of
select-active-regions.

* src/keyboard.c (command_loop_1): Avoid setting selection twice,
since it's done in deactivate-mark as well.
(Vselect_active_regions): Replace `lazy' value with `only',
meaning to only set PRIMARY for temporarily active regions.