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/masterdont/example/masterdont.conf

@@ -0,0 +1,19 @@
+# example config file for Masterdont.
+# this is a comment.
+
+# global server settings.
+server:
+	# port number to use. default 53 (DNS).
+	port: 10053
+
+# a number of zone entries follow.
+zone:
+	# domain name of the zone.
+	name: "example.com"
+	# directory where data is stored for the zone.
+	# In the directory files created are named zone.example.com.*
+	dir: "example"
+
+#zone: 
+	#name: "example.net"
+	#dir: "example"