EVP: Clarify the states of an EVP_PKEY

EVP: Clarify the states of an EVP_PKEY

EVP_PKEY is rather complex, even before provider side keys entered the
stage.
You could have untyped / unassigned keys (pk->type == EVP_PKEY_NONE),
keys that had been assigned a type but no data (pk->pkey.ptr == NULL),
and fully assigned keys (pk->type != EVP_PKEY_NONE && pk->pkey.ptr != NULL).

For provider side keys, the corresponding states weren't well defined,
and the code didn't quite account for all the possibilities.

We also guard most of the legacy fields in EVP_PKEY with FIPS_MODE, so
they don't exist at all in the FIPS module.

Most of all, code needs to adapt to the case where an EVP_PKEY's
|keymgmt| is non-NULL, but its |keydata| is NULL.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11375)