Revise EVP_PKEY param handling

Revise EVP_PKEY param handling

We add new functions for getting parameters and discovering the gettable
and settable parameters. We also make EVP_PKEY_CTX_get_signature_md() a
function and implement it in terms of the new functions.

This enables applications to discover the set of parameters that are
supported for a given algorithm implementation.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9753)