And add names for the curve implemented in crypto/elliptic.

This permits a safer alternative to switching on BitSize
for code that implements curve-dependent cryptosystems.
(E.g., ECDSA on P-xxx curves with the matched SHA-2
instances.)

Change-Id: I653c8f47506648028a99a96ebdff8389b2a95fc1
Reviewed-on: https://go-review.googlesource.com/2133Reviewed-by: Adam Langley <agl@golang.org>