Running strlenpt() on a string that takes up more than 80 columns
(the width of an average terminal) takes /more/ time than simply
converting an extra character (the one that will be overwritten
by the "$" at the edge of the terminal).  So... just convert one
more character than necessary when the line is overlong.  In the
most common case, however, the line will fit fully onscreen, and
we save a whole call of strlenpt().