Add EVP_PKEY_get0_hmac() function

Before the addition of this function, it was impossible to read the
symmetric key from an EVP_PKEY_HMAC type EVP_PKEY.

Reviewed-by: Emilia Käsper <emilia@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1217)