Enlarge radius of box drawings arcs

Enlarge radius of box drawings arcs

Before, the diagonal parts were the same as in the capital O.  But that
was already a bit square, and the shapes one would draw with the box
drawing characters will generally be bigger.  So now I've cut a bit more
off the corners.  This means that arcs that join to the right need the
recent change to add JOIN_R in order to join up properly.

With this change, if you try to draw a circle with them, you get an
octagon with side lengths 2, 2.8, 6 instead of 4, 1.4, 8, which I think
is an improvement.