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/examples/ldns-mx.1

@@ -0,0 +1,23 @@
+.TH ldns-mx 1 "27 Apr 2005"
+.SH NAME
+ldns-mx \- print out the mx record(s) for a domain
+.SH SYNOPSIS
+.B ldns-mx
+.IR DOMAIN 
+
+.SH DESCRIPTION
+\fBldns-mx\fR is used to print out mx information of a domain.
+
+.SH OPTIONS
+\fBldns-mx\fR has no options.
+
+.SH AUTHOR
+Written by the ldns team as an example for ldns usage.
+
+.SH REPORTING BUGS
+Report bugs to <dns-team@nlnetlabs.nl>.
+
+.SH COPYRIGHT
+Copyright (C) 2005 NLnet Labs. This is free software. There is NO
+warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
+PURPOSE.