[statistics] Provide basic statistics for the NAT

Count number of SCCP connections, number of BSC reconnects,
number of calls. For most of them we have a per BSC and a
global count.

Right now all structs using the counters survive until the
end of the application so we do not need to free them.
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index 8897cc3..e679a91 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -258,7 +258,10 @@
 		case SCCP_MSG_TYPE_IT:
 			con = patch_sccp_src_ref_to_bsc(msg, parsed, nat);
 			if (parsed->gsm_type == BSS_MAP_MSG_ASSIGMENT_RQST) {
+				counter_inc(nat->stats.sccp.calls);
+
 				if (con) {
+					counter_inc(con->bsc->cfg->stats.sccp.conn);
 					if (bsc_mgcp_assign(con, msg) != 0)
 						LOGP(DNAT, LOGL_ERROR, "Failed to assign...\n");
 				} else
@@ -466,6 +469,7 @@
 
 	llist_for_each_entry(conf, &bsc->nat->bsc_configs, entry) {
 		if (strcmp(conf->token, token) == 0) {
+			counter_inc(conf->stats.net.reconn);
 			bsc->authenticated = 1;
 			bsc->cfg = conf;
 			bsc_del_timer(&bsc->id_timeout);
@@ -632,6 +636,9 @@
 		return ret;
 	}
 
+	/* count the reconnect */
+	counter_inc(nat->stats.bsc.reconn);
+
 	/*
 	 * if we are not connected to a msc... just close the socket
 	 */