Introduce a VTY, factually turning OpenGGSN into an Osmocom program

Change-Id: I9613ca3436e77ea132c47f0096df7c5050d7e826
diff --git a/examples/osmo-ggsn.cfg b/examples/osmo-ggsn.cfg
new file mode 100644
index 0000000..763e561
--- /dev/null
+++ b/examples/osmo-ggsn.cfg
@@ -0,0 +1,71 @@
+!
+! OpenGGSN (0.94.1-adac) configuration saved from vty
+!!
+!
+log stderr
+  logging filter all 1
+  logging color 1
+  logging print category 0
+  logging timestamp 0
+  logging level ip info
+  logging level tun info
+  logging level ggsn info
+  logging level sgsn notice
+  logging level icmp6 notice
+  logging level lglobal notice
+  logging level llapd notice
+  logging level linp notice
+  logging level lmux notice
+  logging level lmi notice
+  logging level lmib notice
+  logging level lsms notice
+  logging level lctrl notice
+  logging level lgtp info
+  logging level lstats notice
+  logging level lgsup notice
+  logging level loap notice
+  logging level lss7 notice
+  logging level lsccp notice
+  logging level lsua notice
+  logging level lm3ua notice
+  logging level lmgcp notice
+!
+stats interval 5
+!
+line vty
+ no login
+!
+ggsn ggsn0
+ gtp state-dir /tmp
+ gtp bind-ip 127.0.0.6
+ apn internet
+  gtpu-mode tun
+  tun-device tun4
+  type-support v4
+  ip prefix dynamic 176.16.222.0/24
+  ip dns 0 192.168.100.1
+  ip dns 1 8.8.8.8
+  ip ifconfig 176.16.222.0/24
+  no shutdown
+ apn inet6
+  gtpu-mode tun
+  tun-device tun6
+  type-support v6
+  ipv6 prefix dynamic 2001:780:44:2000:0:0:0:0/56
+  ipv6 dns 0 2001:4860:4860::8888
+  ipv6 ifconfig 2001:780:44:2000:0:0:0:0/56
+  no shutdown
+ apn inet46
+  gtpu-mode tun
+  tun-device tun46
+  type-support v4v6
+  ip prefix dynamic 176.16.46.0/24
+  ip dns 0 192.168.100.1
+  ip dns 1 8.8.8.8
+  ip ifconfig 176.16.46.0/24
+  ipv6 prefix dynamic 2001:780:44:2100:0:0:0:0/56
+  ipv6 dns 0 2001:4860:4860::8888
+  ipv6 ifconfig 2001:780:44:2100:0:0:0:0/56
+  no shutdown
+ default-apn internet
+ no shutdown ggsn