payload: Use common prefixes for all payload type identifiers

The old identifiers did not use a proper namespace and often clashed with
other defines.

src/conftest/hooks/set_proposal_number.c

@@ -85,7 +85,7 @@ METHOD(listener_t, message, bool,
 		enumerator = message->create_payload_enumerator(message);
 		while (enumerator->enumerate(enumerator, &payload))
 		{
-			if (payload->get_type(payload) == SECURITY_ASSOCIATION)
+			if (payload->get_type(payload) == PLV2_SECURITY_ASSOCIATION)
 			{
 				sa = (sa_payload_t*)payload;
 				list = sa->get_proposals(sa);