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/drill/README

@@ -0,0 +1,12 @@
+QUICK INSTALL GUIDE
+
+drill is a subdirectory in ldns.
+
+To compile drill you need:
+autoreconf && ./configure && make
+
+If ldns is installed in a different location, use --with-ldns=directory
+See also ./configure --help
+
+In the first case you must run drill as:
+LD_LIBRARY_PATH=../.libs ./drill <options>