USG graphics bugs

[urban at trwspp.UUCP]

   The gtop program (and the hpd device driver which I
mined for another device) uses a routine called 'devarc' to
compute a center and radius for an arc given three points.
This routine core-dumps (division by 0) if the points are
collinear (which happens in thin slices generated by "pie"
sometimes).  This should be fixed to draw a line between
the points, of course.

	Mike