New gui-selection-value consolidating x-selection-value.

New gui-selection-value consolidating x-selection-value.
* lisp/select.el (gui-selection-value-alist): New method.
(gui-selection-value): New function.
(x-selection-value): Make it an obsolete alias.
* lisp/simple.el (interprogram-paste-function): Default to gui-selection-value.
* lisp/w32-common-fns.el (w32-get-selection-value): Simplify.
(x-selection-value): Remove alias.
(interprogram-paste-function): Don't set.
(gui-selection-value): Define for w32.
* lisp/term/x-win.el (gui-selection-value): Define for x.
(x--selection-value): Rename from x--selection-value.
(interprogram-paste-function): Don't set.
* lisp/term/pc-win.el (w16-get-selection-value): Simplify.
(msdos-initialize-window-system): Don't set interprogram-paste-function.
(gui-selection-value): Define for pc.
* lisp/term/ns-win.el (x-selection-value): Remove.
(gui-selection-value): Define for ns, instead.
* lisp/term/common-win.el (x-setup-function-keys): Don't set
interprogram-paste-function.
* lisp/obsolete/mouse-sel.el (mouse-sel-get-selection-function):
Use gui-selection-value.