Prepare pcks12 for opaque structs in libcrypto

Prepare pcks12 for opaque structs in libcrypto

get_cert_chain() needs some error checking. return X509_V_ errors
instead of trying to overload the NULL and then whine in a comment that
this won't really work.

Fix a bug that printed only the first attribute by factoring out the
thing that did the actual printing.

Sprinkle a few changes to accessors here and there.
This is loosely based on what OpenSSL did with some simplifications by
jsing.

ok beck jsing