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

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

> show ns nse 1234

NSEI 01234: UDP, ALIVE since 0d 0h 0m 16s
 FSM Instance Name: 'GPRS-NS2-SNS-SGSN(NSE01234-SNS)[0x6120000012a0]', ID: 'NSE01234-SNS'
  Log-Level: 'DEBUG', State: 'CONFIGURED'
  Timer: 4
 Maximum number of remote  NS-VCs: 8, IPv4 Endpoints: 2, IPv6 Endpoints: 0
 [...]

Related: OS#5028
Change-Id: I8143080a3c5c9a55d37dfad44ba2ac6561daa216
diff --git a/src/gb/gprs_ns2_internal.h b/src/gb/gprs_ns2_internal.h
index afe6b69..cd9969c 100644
--- a/src/gb/gprs_ns2_internal.h
+++ b/src/gb/gprs_ns2_internal.h
@@ -220,6 +220,9 @@
 
 	/*! recursive anchor */
 	bool freed;
+
+	/*! when the NSE became alive or dead */
+	struct timespec ts_alive_change;
 };
 
 /*! Structure representing a single NS-VC */