ascii: Improve look of dashed line ends.

ascii: Improve look of dashed line ends.

Until now, partial ends of horizontal dashed lines were rendered as
U+2574 BOX DRAWINGS LIGHT LEFT and U+2576 BOX DRAWINGS LIGHT RIGHT,
which are solid lines.  This looks funny at the end of a dashed line,
especially in some fonts.  This commit changes these ends to
U+254C BOX DRAWINGS LIGHT DOUBLE DASH HORIZONTAL, which does extend all the
way across the character cell but looks much better.