Prepare to provide EVP_PKEY_{public,param}_check

Prepare to provide EVP_PKEY_{public,param}_check

This implements checking of a public key and of key generation
parameters for DH and EC keys. With the same logic and setters
and const quirks as for EVP_PKEY_check().

There are a couple of quirks: For DH no default EVP_PKEY_check()
is implemented, instead EVP_PKEY_param_check() calls DH_check_ex()
even though DH_param_check_ex() was added for this purpose.
EVP_PKEY_public_check() for EC curves also checks the private key
if present.

ok inoguchi jsing