(x_screen): Make this var file scope.

(x_screen): Make this var file scope.
(Fx_server_version): Use Fcons, not list3.

(Fx_server_vendor, Fx_server_version, Fx_display_pixel_width)
(Fx_display_pixel_height, Fx_display_mm_width, Fx_display_mm_height)
(Fx_display_screens, Fx_display_planes, Fx_display_color_cells)
(Fx_display_visual_class, Fx_display_backing_store)
(Fx_display_save_under): New functions.
(x_screen_count, Vx_vendor, x_release, x_screen_height_mm)
(x_screen_width_mm, Vx_backing_store, x_save_under, Vx_screen_visual)
(x_visual_strings): Vars deleted.
(Fx_open_connection): Don't init those vars.
(syms_of_xfns): Set up new functions.  Don't set up those vars as Lisp vars.
Nor x-screen-width and x-screen-height.