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-rrsig.1

@@ -0,0 +1,30 @@
+.TH ldns-rrsig 1 "27 Apr 2005"
+.SH NAME
+ldns-rrsig \- print out the inception and expiration dates in human
+readable form
+.SH SYNOPSIS
+.B ldns-rrsig 
+.IR domain
+[ 
+.IR type 
+]
+
+.SH DESCRIPTION
+\fBldns-rrsig\fR is used to print the expiration and inception date of
+a RRSIG. The first argument is a domain name. \fBldns-rrsig\fR will
+query the authoritative servers for that domain to get a list of RRSIGs.
+It will then print out the inception and expiration dates for the RRSIG
+covering the SOA record.
+.PP
+If the second argument \fBtype\fR is given the RRSIG covering that type will be shown.
+
+.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.