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/test/29-generic-rdata-tab.tpkg/29-generic-rdata-tab.test

@@ -0,0 +1,15 @@
+# #-- 29-generic-rdata-tab.test --#
+# source the master var file when it's there
+[ -f ../.tpkg.var.master ] && source ../.tpkg.var.master
+
+export PATH=$PATH:/usr/sbin:/sbin:/usr/local/bin:/usr/local/sbin:.
+
+export LD_LIBRARY_PATH=../../lib:$LD_LIBRARY_PATH
+
+OUTPUT=`../../examples/ldns-read-zone 29-generic-rdata-tab.db`
+
+if [[ $? -eq 0 ]]; then
+    echo $OUTPUT | grep CLASS32
+else
+        exit 1
+fi