The pkey_ctrl() method adds extra algorithm specific control.
It's called by L<EVP_PKEY_get_default_digest_nid(3)>,
-L<EVP_PKEY_supports_digest_nid(3)>,
L<EVP_PKEY_set1_encoded_public_key(3)>,
L<EVP_PKEY_get1_encoded_public_key(3)>, L<PKCS7_SIGNER_INFO_set(3)>,
L<PKCS7_RECIP_INFO_set(3)>, ...