ns2: nsvc: add a uptime/downtime to track the last state change

To show adminstrator the last state change of a nsvc add a timestamp and
show it on the vty

> show ns nsei 1234
NSEI 01234: UDP, DEAD since 0d 0h 1m 42s
[...]
  4 NS-VC:
   UNBLOCKED DYNAMIC sig_weight=1 data_weight=1 udp)[127.0.0.1]:22000<>[127.0.0.1]:23001 ALIVE since 0d 0h 0m 1s
   UNBLOCKED DYNAMIC sig_weight=2 data_weight=2 udp)[127.0.0.1]:22000<>[127.0.0.1]:23000 ALIVE since 0d 0h 0m 1s
   UNBLOCKED DYNAMIC sig_weight=2 data_weight=2 udp)[127.0.0.1]:22001<>[127.0.0.1]:23000 ALIVE since 0d 0h 0m 1s
   UNBLOCKED DYNAMIC sig_weight=1 data_weight=1 udp)[127.0.0.1]:22001<>[127.0.0.1]:23001 ALIVE since 0d 0h 0m 1s

Related: OS#5028
Change-Id: Ie3a039a209869295afa5feda39297cee81fedf22
diff --git a/src/gb/gprs_ns2_internal.h b/src/gb/gprs_ns2_internal.h
index cd9969c..bfb12d9 100644
--- a/src/gb/gprs_ns2_internal.h
+++ b/src/gb/gprs_ns2_internal.h
@@ -270,6 +270,9 @@
 
 	/*! recursive anchor */
 	bool freed;
+
+	/*! when the NSVC became alive or dead */
+	struct timespec ts_alive_change;
 };
 
 /*! Structure repesenting a bind instance. E.g. IPv4 listen port. */