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/14-read-zone.tpkg/14-read-zone.db

@@ -0,0 +1,23 @@
+;
+; BIND data file for miek.nl for internal use
+;
+$TTL    1H
+@       IN      SOA     elektron.atoom.net. miekg.atoom.net. (
+                     2005060700         ; Serial
+                             6H         ; Refresh
+                             2H         ; Retry
+                             7D         ; Expire
+                             1H )       ; Negative Cache TTL
+
+@       IN      NS      elektron.atoom.net.
+@	IN	MX	10 elektron.atoom.net.
+@	IN	A	192.168.1.2
+
+a	IN	A	192.168.1.2
+www     IN      CNAME   a
+; hallo
+
+
+;
+www     in      A      127.0.0.1
+; bla