A new method is added to EVP_PKEY_METH as:

    int (*check) (EVP_PKEY_CTX *ctx);

and to EVP_PKEY_ASN1_METHOD as:

    int (*pkey_check) (EVP_PKEY_CTX *ctx);

This is used to check the validity of a specific key.

The order of calls is:
EVP_PKEY_check -> pmeth.check -> ameth.pkey_check.
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4337)