fix: populate ldns submodule and add autotools to LDNS build stage

- Re-cloned zonemaster-ldns with --recurse-submodules so the bundled
  ldns C library source (including Changelog and configure.ac) is present
- Added autoconf, automake, libtool to Dockerfile.backend ldns-build stage
  so libtoolize + autoreconf can generate ldns/configure during make

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

zonemaster-ldns/ldns/lua/serv.lua

@@ -0,0 +1,63 @@
+-- source the lib file with the function
+dofile("rns-lib.lua")
+
+rr1 = record.new_frm_str("www.miek.nl  IN A 192.168.1.2", 0, nil)
+rr2 = record.new_frm_str("miek.nl  IN ns gaap", 0, nil)
+rr3 = record.new_frm_str("miek.nl  IN ns gaap2", 0, nil)
+rr4 = record.new_frm_str("www.atoom.net. IN A 192.168.1.2", 0, nil)
+rr5 = record.new_frm_str("www.nlnetlabs.nl IN A 192.168.1.2", 0, nil)
+rr6 = record.new_frm_str("www.nlnet.nl IN A 192.168.1.2", 0, nil)
+
+pkt = packet.new()
+pkt = packet.push_rr(pkt, LDNS_SECTION_ANSWER, rr1)
+pkt = packet.push_rr(pkt, LDNS_SECTION_ANSWER, rr4)
+pkt = packet.push_rr(pkt, LDNS_SECTION_AUTHORITY, rr2)
+pkt = packet.push_rr(pkt, LDNS_SECTION_AUTHORITY, rr3)
+
+---- Setup a server to listen to UDP -- bit strange to first
+-- make a rdf out of it and then continue with the sockaddr struct
+rdf_ip = rdf.new_frm_str(LDNS_RDF_TYPE_A, "127.0.0.1")
+socket = udp.server_open(rdf_ip, 5353)
+if socket == nil then
+        os.exit(EXIT_FAILURE)
+end
+
+
+while true do
+	-- read from the socket, this blocks...
+	wirebuf, sockaddr_from  = udp.read(socket)
+
+	-- wrap this in new functions
+	if wirebuf == nil then
+		lua_debug("nothing received")
+	else
+		-- somebody is writing
+		wirepkt = buffer.to_pkt(wirebuf)
+
+		lua_debug("received from the interface")
+
+		-- next we must send it to our recursive nameserver
+		-- and pick up the result
+		-- then we modify the result somewhat and sent it back
+		-- to the client
+
+		id = packet.id(wirepkt);
+		packet.print(wirepkt)
+
+		-- set the id on the outgoing packet
+		packet.set_id(pkt, id)
+		lua_packet_ancount_incr(pkt, 2)
+		wirebuf2 = packet.to_buf(pkt)
+
+		-- write back to the client
+		bytes = lua_udp_write(socket, wirebuf2, sockaddr_from)
+		if bytes == -1 then
+			lua_debug("write error")
+		else 
+			lua_debug("wrote bytes", bytes)
+			packet.print(pkt)
+		end
+		
+	end
+end
+udp.close(socket)