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>

Converting check_autoarg from a macro to a function lost the behavior. Instead,
just move the logic into p_rsa.c which was the only EVP_PKEY implementation
that even needed the flag.

Also document this behavior on each of the functions. Make note of the out =
NULL case only returning the maximum output size, and not necessarily the
actual size.

For testing, update example_sign to determine the signature size using the NULL
behavior rather than querying the RSA key.

Change-Id: Iec6c2862028a5cfdefe8faa0e8c471755070898a
Reviewed-on: https://boringssl-review.googlesource.com/1121
Reviewed-by: Adam Langley <agl@google.com>

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)