Add support for the french ANSSI FRP256v1 elliptic curve.

Add support for the french ANSSI FRP256v1 elliptic curve.

While not to be considered a good choice of elliptic curve (refer to
http://safecurves.cr.yp.to/ for more details), it is nevertheless deemed a
good decision to allow developers with requirements to use such a curve,
to be able to do this via a crypto library allowing for much better choices
to be made, without having to change (much of) their code to get better crypto.

ok beck@ deraadt@