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/test.lua

@@ -0,0 +1,32 @@
+-- source the lib file with the function
+dofile("rns-lib.lua")
+
+-- Now the scary ldns_* stuff
+my_rr = record.new_frm_str("www.miek.nl  IN A 192.168.1.2")
+my_rr2 = record.new_frm_str("www.miek.nl")
+my_rr4 = record.new_frm_str("www.atoom.net. IN A 192.168.1.2")
+
+record.print(my_rr)
+record.print(my_rr2)
+record.print(my_rr4)
+
+my_pkt = packet.new();
+
+my_pkt = packet.push_rr(my_pkt, LDNS_SECTION_ANSWER, my_rr)
+
+packet.print(my_pkt)
+
+my_pkt = packet.push_rr(my_pkt, LDNS_SECTION_ANSWER, my_rr2)
+
+my_rr3 = packet.get_rr(my_pkt, 0);
+record.print(my_rr3)
+my_rr3 = packet.get_rr(my_pkt, 1);
+record.print(my_rr3)
+
+packet.print(my_pkt)
+my_rr5 = packet.set_rr(my_pkt, my_rr4, 1)
+record.print(my_rr5)
+
+packet.set_id(my_pkt, 1505)
+
+packet.print(my_pkt)