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/contrib/DNS-LDNS/lib/DNS/LDNS/RBNode.pm

@@ -0,0 +1,71 @@
+package DNS::LDNS::RBNode;
+
+use 5.008008;
+use strict;
+use warnings;
+
+use DNS::LDNS;
+
+our $VERSION = '0.61';
+
+# Note: This class does not have a constructor. Thus, it can not be created
+# as an individual object. The data structure of the object will always be 
+# owned and freed by its parent object.
+
+sub next {
+    my $self = shift;
+    return DNS::LDNS::GC::own($self->_next, $self);
+}
+
+sub previous {
+    my $self = shift;
+    return DNS::LDNS::GC::own($self->_previous, $self);
+}
+
+sub next_nonglue {
+    my $self = shift;
+    return DNS::LDNS::GC::own($self->_next_nonglue, $self);
+}
+
+sub name {
+    my ($self) = @_;
+    return DNS::LDNS::GC::own($self->_name, $self);
+}
+
+sub DESTROY {
+    DNS::LDNS::GC::free($_[0]);
+}
+
+1;
+__END__
+
+=head1 NAME
+
+DNS::LDNS::RBNode - Node in the RBTree
+
+=head1 SYNOPSIS
+
+  use DNS::LDNS ':all'
+
+  node2 = node->next
+  node2 = node->next_nonglue
+  bool = node->is_null
+  dnssec_name = node->name
+
+=head1 SEE ALSO
+
+http://www.nlnetlabs.nl/projects/ldns
+
+=head1 AUTHOR
+
+Erik Pihl Ostlyngen, E<lt>erik.ostlyngen@uninett.noE<gt>
+
+=head1 COPYRIGHT AND LICENSE
+
+Copyright (C) 2013 by UNINETT Norid AS
+
+This library is free software; you can redistribute it and/or modify
+it under the same terms as Perl itself, either Perl version 5.14.2 or,
+at your option, any later version of Perl 5 you may have available.
+
+=cut