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/ldnsx/examples/ldnsx-walk.py

@@ -0,0 +1,25 @@
+#!/usr/bin/python
+# vim:fileencoding=utf-8
+#
+# Walk a domain that's using NSEC and print in zonefile format.
+
+import sys
+import ldnsx
+
+def walk(domain):
+        res = ldnsx.resolver("193.110.157.136", dnssec=True)
+        pkt = res.query(domain, 666)
+        try:
+                nsec_rr = pkt.authority(rr_type="NSEC")[0]
+        except:
+                print "no NSEC found, domain is not signed or using NSEC3"
+                sys.exit()
+        for rr_type in nsec_rr[5].split(' ')[:-1]:
+                for rr in ldnsx.get_rrs(domain, rr_type):
+                        print str(rr)[:-1]
+        next_rec = nsec_rr[4]
+        if (next_rec != domain) and (next_rec[-len(domain):] == domain):
+                walk(next_rec)
+
+walk("xelerance.com")
+