Added missing signature algorithm reflection functions

    SSL_get_signature_nid()      -- local signature algorithm
    SSL_get_signature_type_nid() -- local signature algorithm key type
    SSL_get_peer_tmp_key()       -- Peer key-exchange public key
    SSL_get_tmp_key              -- local key exchange public key

Aliased pre-existing SSL_get_server_tmp_key(), which was formerly
just for clients, to SSL_get_peer_tmp_key().  Changed internal
calls to use the new name.

Reviewed-by: Matt Caswell <matt@openssl.org>