Custom RSA and ECDSA keys may not expose the key material. Plumb and "opaque"
bit out of the *_METHOD up to EVP_PKEY. Query that in ssl_rsa.c to skip the
sanity checks for certificate and key matching.

Change-Id: I362a2d5116bfd1803560dfca1d69a91153e895fc
Reviewed-on: https://boringssl-review.googlesource.com/1255
Reviewed-by: Adam Langley <agl@google.com>