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/contrib/python/examples/python3/ldns-newpkt.py

@@ -0,0 +1,17 @@
+#!/usr/bin/python
+
+import ldns
+
+pkt = ldns.ldns_pkt.new_query_frm_str("www.google.com",ldns.LDNS_RR_TYPE_ANY, ldns.LDNS_RR_CLASS_IN, ldns.LDNS_QR | ldns.LDNS_AA)
+
+rra = ldns.ldns_rr.new_frm_str("www.google.com. IN A 192.168.1.1",300)
+rrb = ldns.ldns_rr.new_frm_str("www.google.com. IN TXT Some\ Description",300)
+
+list = ldns.ldns_rr_list()
+if (rra): list.push_rr(rra)
+if (rrb): list.push_rr(rrb)
+
+pkt.push_rr_list(ldns.LDNS_SECTION_ANSWER, list)
+
+print("Packet:")
+print(pkt)