keep some internal statistics inside OpenBSC

the statistics will give us some idea about the network load and
performance.
diff --git a/openbsc/src/paging.c b/openbsc/src/paging.c
index b273419..538e0a8 100644
--- a/openbsc/src/paging.c
+++ b/openbsc/src/paging.c
@@ -212,6 +212,8 @@
 	cbfn = req->cbfn;
 	paging_remove_request(&req->bts->paging, req);
 
+	req->bts->network->stats.paging.expired++;
+
 	dispatch_signal(SS_PAGING, S_PAGING_COMPLETED, &sig_data);
 	if (cbfn)
 		cbfn(GSM_HOOK_RR_PAGING, GSM_PAGING_EXPIRED, NULL, NULL,
@@ -254,6 +256,8 @@
 	struct gsm_bts *bts = NULL;
 	int num_pages = 0;
 
+	network->stats.paging.attempted++;
+
 	/* start paging subscriber on all BTS within Location Area */
 	do {
 		int rc;
@@ -269,6 +273,9 @@
 			return rc;
 	} while (1);
 
+	if (num_pages == 0)
+		network->stats.paging.detached++;
+
 	return num_pages;
 }