D-GSM 2/n: implement mDNS method of mslookup server

Implement the mslookup server's mDNS responder, to actually service remote
mslookup requests:
- VTY mslookup/server config with service names,
- the mslookup_mdns_server listening for mslookup requests,

For a detailed overview of the D-GSM and mslookup related files, please see the
elaborate comment at the top of mslookup.c (already added in an earlier patch).

Change-Id: I5cae6459090588b4dd292be90a5e8903432669d2
diff --git a/tests/test_nodes.vty b/tests/test_nodes.vty
index c056e4d..ee351e3 100644
--- a/tests/test_nodes.vty
+++ b/tests/test_nodes.vty
@@ -15,6 +15,7 @@
   show gsup-connections
   subscriber (imsi|msisdn|id|imei) IDENT show
   show subscriber (imsi|msisdn|id|imei) IDENT
+  show mslookup services
 
 OsmoHLR> enable
 OsmoHLR# ?
@@ -25,9 +26,11 @@
 OsmoHLR(config)# ?
 ...
   hlr       Configure the HLR
+  mslookup  Configure Distributed GSM mslookup
 OsmoHLR(config)# list
 ...
   hlr
+  mslookup
 
 OsmoHLR(config)# hlr
 OsmoHLR(config-hlr)# ?