X25519 (RFC 7748) is now commonly used for key agreement in TLS
connections, as specified in
https://tools.ietf.org/html/draft-ietf-tls-curve25519-01.

This change adds support for that in crypto/tls, but does not enabled it
by default so that there's less test noise. A future change will enable
it by default and will update all the test data at the same time.

Change-Id: I91802ecd776d73aae5c65bcb653d12e23c413ed4
Reviewed-on: https://go-review.googlesource.com/30824Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>