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/main.c

@@ -0,0 +1,48 @@
+#include "config.h"
+#include "config_file.h"
+#include "zones.h"
+#include "server.h"
+
+static void
+usage(const char* me)
+{
+	printf("usage: %s [options]\n", me);
+	printf("Hidden master stealth server: serves AXFR, IXFR.\n");
+	printf("-h		print this information.\n");
+	printf("-c <file>	set config file to use.\n");
+	printf("\n");
+}
+
+int main(int argc, char* argv[])
+{
+	const char* config = DEFAULT_CONFIG;
+	int c;
+
+	while((c=getopt(argc, argv, "c:h")) != -1)
+	{
+		switch(c) {
+		case 'c':
+			config = optarg;
+			break;
+		default:
+			printf("Unknown option '-%c' (%x).\n", c, c);
+		case 'h':
+			usage(argv[0]);
+			return 1;
+		}
+	}
+	argc -= optind;
+	argv += optind;
+	if(argc > 0) {
+		printf("Too many arguments.\n");
+		usage(argv[0]);
+		return 1;
+	}
+	
+	/* start server */
+	while(server_start(config)) {
+		printf("Masterdont reload\n");
+	}
+	printf("Masterdont stopped\n");
+	return 0;
+}