[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/paging.c b/src/paging.c
index 761facd..c7fccad 100644
--- a/src/paging.c
+++ b/src/paging.c
@@ -185,12 +185,15 @@
 		req, req->subscr->imsi);
 	
 	struct paging_signal_data sig_data = {
+		.data = {
+			.area = S_PAGING,
+		},
 		.subscr = req->subscr,
 		.bts	= req->bts,
 		.lchan	= NULL,
 	};
 
-	dispatch_signal(S_PAGING, &sig_data.data);
+	dispatch_signal(&sig_data.data);
 	paging_remove_request(&req->bts->paging, req);
 }