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.dsc

@@ -0,0 +1,16 @@
+BaseName: 29-generic-rdata-tab
+Version: 1.0
+Description: Check for GH 157 regression
+CreationDate: Tue Mar 15 21:01:34 EST 2022
+Maintainer: 
+Category: 
+Component:
+CmdDepends: 
+Depends: 
+Help: 29-generic-rdata-tab.help
+Pre: 
+Post: 
+Test: 29-generic-rdata-tab.test
+AuxFiles: 
+Passed:
+Failure: