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/doc/TODO

@@ -0,0 +1,14 @@
+TODO
+
+Features:
+* Drill: print appropriate DS RRs (relates to Bug #355)
+* ldns-signzone optout to be really optout
+* Compression when generating wireformat. Preferably with a configurable 
+  policy: refer only backwards, case sensitivity, only refer to other owner 
+  names, etc.
+
+Bugfixes:
+* Bug #279: fix return values for net.h functions, and related: make return
+  values for functions that cannot return memory-failure today. Needs medium
+  version increase because of API change.
+* Lazy ABI