Implement wave-style variant of underlining.

Implement wave-style variant of underlining.

  * doc/lispref/display.texi: Document new face attribute.
  * lisp/cus-face.el (custom-face-attributes): Add wave-style underline
  attribute.
  * lisp/faces.el (set-face-attribute): Update docstring.
  * src/dispextern.h (face_underline_type): New enum.
  (face): Add field for underline type.
  * src/nsterm.m (ns_draw_underwave): New function.
  (ns_draw_text_decoration): Use it.
  * src/w32term.c (w32_restore_glyph_string_clip, w32_draw_underwave):
  New functions.
  (x_draw_glyph_string): Use them.
  * src/xfaces.c (Qline, Qwave): New Lisp objects.
  (check_lface_attrs, merge_face_ref)
  (Finternal_set_lisp_face_attribute, realize_x_face): Handle
  wave-style underline face attributes.
  * src/xterm.c (x_draw_underwave): New function.
  (x_draw_glyph_string): Use it.