Change clipboard/primary selection to X application standards.

Change clipboard/primary selection to X application standards.

* lisp/menu-bar.el (menu-bar-enable-clipboard): Don't overwrite
Cut/Copy/Paste menu bar items.

* lisp/mouse.el: Bind mouse-2 to mouse-yank-primary.
(mouse-drag-copy-region): Default to nil.

* lisp/simple.el (select-active-regions): Default to t.
(push-mark-command): Don't overwrite primary with empty string.

* lisp/term/x-win.el (x-select-enable-clipboard): Default to t.
(x-initialize-window-system): Don't overwrite Paste menu item.