Currently an ECDHE handshake uses the client's curve preference. This
generally means that we use P-521. However, P-521's strength is
mismatched with the rest of the cipher suite in most cases and we have
a fast, constant-time implementation of P-256.

With this change, Go servers will use P-256 where the client supports
it although that can be overridden in the Config.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/66060043