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/m

@@ -0,0 +1,3 @@
+#!/usr/bin/env bash
+
+grep -v -E '^#' ../doc/function_manpages | sed 's/, /\n/g' | sed 's/|.*$//' | sort | uniq