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/src/Makefile.am b/src/Makefile.am
index ab7a713..093db8e 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -54,8 +54,10 @@
 	gsup_send.c \
 	hlr_ussd.c \
 	lu_fsm.c \
-	mslookup_server.c \
 	timestamp.c \
+	mslookup_server.c \
+	mslookup_server_mdns.c \
+	dgsm_vty.c \
 	$(NULL)
 
 osmo_hlr_LDADD = \