[signal] Move the area into the struct signal_data...

Remove the parameter and move the signal kind into the
signal struct. Make register/deregister fully symmetric.
diff --git a/src/gsm_04_08.c b/src/gsm_04_08.c
index 6776632..4d77d5d 100644
--- a/src/gsm_04_08.c
+++ b/src/gsm_04_08.c
@@ -701,11 +701,14 @@
 		subscr->name ? subscr->name : subscr->imsi);
 
 	struct paging_signal_data sig_data = {
+		.data = {
+			.area = S_PAGING,
+		},
 		.subscr = subscr,
 		.bts	= msg->lchan->ts->trx->bts,
 		.lchan	= msg->lchan,
 	};
-	dispatch_signal(S_PAGING, &sig_data.data);
+	dispatch_signal(&sig_data.data);
 	paging_request_stop(msg->trx->bts, subscr);
 
 	if (!msg->lchan->subscr)