[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/vty_interface_cmds.c b/openbsc/src/vty_interface_cmds.c
index 623aa30..8ade518 100644
--- a/openbsc/src/vty_interface_cmds.c
+++ b/openbsc/src/vty_interface_cmds.c
@@ -233,6 +233,9 @@
 	vty_out(vty, "Channel Requests        : %lu total, %lu no channel%s",
 		counter_get(net->stats.chreq.total),
 		counter_get(net->stats.chreq.no_channel), VTY_NEWLINE);
+	vty_out(vty, "Channel Failures        : %lu rf_failures, %lu rll failures%s",
+		counter_get(net->stats.chan.rf_fail),
+		counter_get(net->stats.chan.rll_err), VTY_NEWLINE);
 	vty_out(vty, "Paging                  : %lu attempted, %lu complete, %lu expired%s",
 		counter_get(net->stats.paging.attempted),
 		counter_get(net->stats.paging.completed),