(ns-handle-switch): Simplify. Handle the numeric case.

(ns-handle-switch): Simplify.  Handle the numeric case.
(ns-handle-numeric-switch): Just call ns-handle-switch.
(ns-handle-name-switch, ns-handle-nxopen, ns-handle-nxopentemp)
(ns-handle-args): Simplify using `pop'.
(ns-display-name): Define (used in frame.el).
(menu-bar-select-frame): Add (ignored) arg to more closely match the
original definition.
(ns-perform-service): Declare.
(ns-save-preferences): Use fewer `let's.