Allow controlling the underline position of faces

Allow controlling the underline position of faces

* doc/lispref/display.texi (Face Attributes): Document new
`:position' property of the `:underline' attribute.
* etc/NEWS: Announce new property.
* lisp/cus-face.el (custom-face-attributes): Implement
customization for new face attribute.
* src/dispextern.h (struct face): New fields
`underline_pixels_above_descent_line' and
`underline_at_descent_line_p'.
* src/haikuterm.c (haiku_draw_text_decoration):
* src/nsterm.m (ns_draw_text_decoration):
* src/w32term.c (w32_draw_glyph_string):
* src/xterm.c (x_draw_glyph_string): Respect new face fields.

* src/xfaces.c (realize_gui_face): Handle new `:position'
keyword.
(syms_of_xfaces): New symbol `:position'.