EVP: add EVP_PKEY_is_a() and EVP_PKEY_can_sign()

EVP: add EVP_PKEY_is_a() and EVP_PKEY_can_sign()

EVP_PKEY_is_a() is the provider side key checking function corresponding
to checking EVP_PKEY_id() or an EVP_PKEY against macros like EVP_PKEY_EC.
It also works with legacy internal keys.

We also add a warning indoc/man3/EVP_PKEY_set1_RSA.pod regarding the
reliability of certain functions that only understand legacy keys.

Finally, we take the opportunity to clean up doc/man3/EVP_PKEY_set1_RSA.pod
to better conform with man-page layout norms, see man-pages(7) on Linux.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11358)