After the custom key method support, the EVP_PKEY parameter is somewhat
confusing (to be resolved with the certificate slots removal) as it must
always refer to a private key. ssl3_cert_verify_hash is sometimes used
with the peer's public key. If custom keys were supported on the server,
this would break.

Fix this by passing a pkey_type parameter and letting the caller decide
whether this uses SSL_PRIVATE_KEY_METHOD or not.

Change-Id: I673b92579a84b4561f28026ec0b1c78a6bfee440
Reviewed-on: https://boringssl-review.googlesource.com/5341
Reviewed-by: Adam Langley <agl@google.com>