curveopt: rename from helixish

We're going to want to reuse the findcurve invocation machinery.  Most
of the rest of this is specific to the abandoned helixish approach, so
is due to be hacked away.  For now just rename this unused file.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

diff --git a/helixish.py b/curveopt.py
similarity index 100%
rename from helixish.py
rename to curveopt.py