Improve window dividers code.

Improve window dividers code.

* faces.el (window-divider): New default value.  Rewrite
doc-string.
(window-divider-first-pixel, window-divider-last-pixel): New
faces.
* dispextern.h (face_id): Add WINDOW_DIVIDER_FIRST_PIXEL_FACE_ID
and WINDOW_DIVIDER_LAST_PIXEL_FACE_ID.
* w32term.c (w32_draw_window_divider): Handle first and last
pixels specially.
* w32term.h (w32_fill_area_abs): New function.
* xdisp.c (x_draw_right_divider): Don't draw over bottom
divider.
* xfaces.c (realize_basic_faces): Handle new face ids.
* xfns.c (Fx_create_frame): Call x_default_parameter for right
and bottom divider width.
* xterm.c (x_draw_window_divider): Handle first and last pixels
specially.