Implement strike-through and overline on NextStep (Bug#8863).

Implement strike-through and overline on NextStep (Bug#8863).

* src/nsfont.m (nsfont_open): Use underline position provided by font,
instead of hard-coded value of 2.
(nsfont_draw): Call ns_draw_text_decoration instead.

* src/nsterm.h: Add declaration for ns_draw_text_decoration.

* src/nsterm.m (ns_draw_text_decoration): New function for drawing
underline, overline, and strike-through.
(ns_dumpglyphs_image, ns_dumpglyphs_stretch): Add call to
ns_draw_text_decoration.  Change treatment of cursor drawing to
accomodate underlining, etc.