  This is a facsimile of the circa 1990 unpublished manuscript with the same
title. All the original text, figures and tables are included; although text
has been reset in \TeX, the original hand-drawn figures have been redrawn
digitally, and the parameter $k$ in the original table of lens spaces has been
replaced with the originally intended $\lambda$. And, of course, this abstract
has been added.
