Add SSL_get_peer_signature_type_nid() function.

Add function to retrieve signature type: in the case of RSA
keys the signature type can be EVP_PKEY_RSA or EVP_PKEY_RSA_PSS.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2301)