(Vtruncate_partial_width_windows): New Lisp_Object, replacing

(Vtruncate_partial_width_windows): New Lisp_Object, replacing
truncate_partial_width_windows.
(init_iterator): If Vtruncate_partial_width_windows is an integer,
truncate only if the window width is below that integer.
(start_display. resize_mini_window, produce_stretch_glyph)
(display_string, move_it_in_display_line_to): Use line_wrap.
(back_to_previous_visible_line_start, reseat_1): Reset
string_from_display_prop_p.
(display_line): Extend default face to end of line when wrapping.
(display_line, move_it_in_display_line_to): Add ability
to wrap continued lines at word boundaries.