[statistics] Keep track of rf failures and rll release failures

Add two new counters to count the RF Failures and the RLL Release
failure and make them available via the vty interface.
diff --git a/openbsc/src/abis_rsl.c b/openbsc/src/abis_rsl.c
index 343347a..bbce67a 100644
--- a/openbsc/src/abis_rsl.c
+++ b/openbsc/src/abis_rsl.c
@@ -810,6 +810,7 @@
 
 	LOGPC(DRSL, LOGL_NOTICE, "\n");
 	/* FIXME: only free it after channel release ACK */
+	counter_inc(msg->lchan->ts->trx->bts->network->stats.chan.rf_fail);
 	return rsl_rf_chan_release(msg->lchan);
 }
 
@@ -1245,8 +1246,10 @@
 
 	rll_indication(msg->lchan, rllh->link_id, BSC_RLLR_IND_ERR_IND);
 
-	if (rlm_cause[1] == RLL_CAUSE_T200_EXPIRED)
+	if (rlm_cause[1] == RLL_CAUSE_T200_EXPIRED) {
+		counter_inc(msg->lchan->ts->trx->bts->network->stats.chan.rll_err);
 		return rsl_rf_chan_release(msg->lchan);
+	}
 
 	return 0;
 }