gtphub: ares vty and init

From sgsn_vty.c, copy the cfg_grx_ggsn_cmd to add an ares server to the static
sgsn_instance.

This is sort of preliminary. As described in comments, the sgsn_ares functions
should actually be separated from the static sgsn structure. gtphub keeps such
an sgsn structure just for the sgsn_ares functions.

Sponsored-by: On-Waves ehi
diff --git a/openbsc/doc/examples/osmo-gtphub/osmo-gtphub.cfg b/openbsc/doc/examples/osmo-gtphub/osmo-gtphub.cfg
index c9bb4cf..0dc4150 100644
--- a/openbsc/doc/examples/osmo-gtphub/osmo-gtphub.cfg
+++ b/openbsc/doc/examples/osmo-gtphub/osmo-gtphub.cfg
@@ -21,3 +21,5 @@
  ! Proxy with nonstandard ports or separate IPs:
  !ggsn-proxy ctrl 127.0.0.3 2123 user 127.0.0.5 2152
 
+ ! Add a name server for GGSN resolution
+ !grx-dns-add 192.168.0.1