We don't actually have an API to let you know if the value is legal to
interpret as a curve ID. (This was kind of a poor API. Oh well.) Also add tests
for key_exchange_info. I've intentionally left server-side plain RSA missing
for now because the SSL_PRIVATE_KEY_METHOD abstraction only gives you bytes and
it's probably better to tweak this API instead.

(key_exchange_info also wasn't populated on the server, though due to a
rebasing error, that fix ended up in the parent CL. Oh well.)

Change-Id: I74a322c8ad03f25b02059da7568c9e1a78419069
Reviewed-on: https://boringssl-review.googlesource.com/6783
Reviewed-by: Adam Langley <agl@google.com>