Fix menu-set-font interaction with Custom themes.

Fix menu-set-font interaction with Custom themes.

In particular, prevent it from setting non-font-related attributes
like the foreground and background color.  This requires a bugfix to
face-spec-reset-face to make "resetting" the default face work.

* lisp/faces.el (face-spec-reset-face): Don't apply unspecified
attribute values to the default face.

* lisp/frame.el (set-frame-font): New arg ALL-FRAMES.

* lisp/menu-bar.el (menu-set-font): Use set-frame-font.