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/ldns-keygen.py

@@ -0,0 +1,46 @@
+#!/usr/bin/python
+#
+# This example shows how to generate public/private key pair
+#
+import ldns
+
+algorithm = ldns.LDNS_SIGN_DSA
+bits = 512
+
+ldns.ldns_init_random(open("/dev/urandom","rb"), (bits+7)//8)
+
+domain = ldns.ldns_dname("example.")
+
+#generate a new key
+key = ldns.ldns_key.new_frm_algorithm(algorithm, bits);
+print key
+
+#set owner
+key.set_pubkey_owner(domain)
+
+#create the public from the ldns_key
+pubkey = key.key_to_rr()
+#previous command is equivalent to
+# pubkey = ldns.ldns_key2rr(key)
+print pubkey
+
+#calculate and set the keytag
+key.set_keytag(ldns.ldns_calc_keytag(pubkey))
+
+#build the DS record
+ds = ldns.ldns_key_rr2ds(pubkey, ldns.LDNS_SHA1)
+print ds
+
+owner, tag = pubkey.owner(), key.keytag()
+
+#write public key to .key file
+fw = open("key-%s-%d.key" % (owner,tag), "wb")
+pubkey.print_to_file(fw)
+
+#write private key to .priv file
+fw = open("key-%s-%d.private" % (owner,tag), "wb")
+key.print_to_file(fw)
+
+#write DS to .ds file
+fw = open("key-%s-%d.ds" % (owner,tag), "wb")
+ds.print_to_file(fw)