ns: Add statistics for some events

The following counters are added to the ns.nsvc counter group:
  lost.alive             The number of missing ALIVE ACK messages
  lost.reset             The number of missing RESET ACK messages

The following items are added to the ns.nsvc stat item group:
  alive.delay            The time in ms between sending ALIVE and
                         receiving the next ALIVE ACK

Sponsored-by: On-Waves ehf
diff --git a/include/osmocom/gprs/gprs_ns.h b/include/osmocom/gprs/gprs_ns.h
index d5a605d..42e0d88 100644
--- a/include/osmocom/gprs/gprs_ns.h
+++ b/include/osmocom/gprs/gprs_ns.h
@@ -118,6 +118,7 @@
 
 	struct osmo_timer_list timer;
 	enum nsvc_timer_mode timer_mode;
+	struct timeval timer_started;
 	int alive_retries;
 
 	unsigned int remote_end_is_sgsn:1;
@@ -125,6 +126,7 @@
 	unsigned int nsvci_is_valid:1;
 
 	struct rate_ctr_group *ctrg;
+	struct stat_item_group *statg;
 
 	/*! \brief which link-layer are we based on? */
 	enum gprs_ns_ll ll;