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/unencrypted_notify.c

@@ -83,7 +83,7 @@ METHOD(listener_t, ike_updown, bool,
 		{
 			data = chunk_clone(chunk_create(this->data, strlen(this->data)));
 		}
-		notify = notify_payload_create_from_protocol_and_type(NOTIFY,
+		notify = notify_payload_create_from_protocol_and_type(PLV2_NOTIFY,
 									this->esp ? PROTO_ESP : PROTO_IKE, type);
 		notify->set_spi(notify, this->spi);
 		if (data.len)