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/include/osmocom/hlr/Makefile.am b/include/osmocom/hlr/Makefile.am
index 5c96ec8..b24f084 100644
--- a/include/osmocom/hlr/Makefile.am
+++ b/include/osmocom/hlr/Makefile.am
@@ -11,6 +11,7 @@
 	logging.h \
 	lu_fsm.h \
 	mslookup_server.h \
+	mslookup_server_mdns.h \
 	rand.h \
 	timestamp.h \
 	$(NULL)