(XTread_socket): Pass keys in menu bar to toolkit alone

(XTread_socket): Pass keys in menu bar to toolkit alone
only for Motif.

(XTread_socket): Fixing previous change:
Clear last_mouse_press_frame on non-menu-bar ButtonPress.
Always use last_mouse_press_frame as frame for ButtonRelease.
But ignore the event if frame's output_data.x is null.
Save the ButtonRelease only if USE_MOTIF.
(last_mouse_press_frame): This is now a Lisp object.
(syms_of_xterm): Initialize and staticpro it.