Previously we only needed to be able to serve P-224 certificates, but
now we anticipate a need to be able to connect and validate them also.
Since this requires advertising support for P-224 in the handshake, we
need to support P-224 ECDHE too.

P-224 support is disabled by default and so clients need to both set the
enabled curves explicitly and set a maximum version of TLS 1.2.

Change-Id: Idc69580f47334e0912eb431a0db0e78ee2eb5bbe
Reviewed-on: https://boringssl-review.googlesource.com/14225
Reviewed-by: Adam Langley <alangley@gmail.com>
Commit-Queue: Adam Langley <alangley@gmail.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>