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

The renegotiation extension was introduced[1] due to an attack by Ray in
which a client's handshake was spliced into a connection that was
renegotiating, thus giving an attacker the ability to inject an
arbitary prefix into the connection.

Go has never supported renegotiation as a server and so this attack
doesn't apply. As a client, it's possible that at some point in the
future the population of servers will be sufficiently updated that
it'll be possible to reject connections where the server hasn't
demonstrated that it has been updated to address this problem.

We're not at that point yet, but it's good for Go servers to support
the extension so that it might be possible to do in the future.

[1] https://tools.ietf.org/search/rfc5746

R=golang-codereviews, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/48580043

The practice of storing reference connections for testing has worked
reasonably well, but the large blocks of literal data in the .go files
is ugly and updating the tests is a real problem because their number
has grown.

This CL changes the way that reference tests work. It's now possible to
automatically update the tests and the test data is now stored in
testdata/. This should make it easier to implement changes that affect
all connections, like implementing the renegotiation extension.

R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/42060044