Merge #1992

1992: Improve unbound configuration r=mergify[bot] a=nextgens

## What type of PR?

enhancement

## What does this PR do?

Improve the unbound configuration:
- remove settings that were default
- don't setup IPv6 if it's not configured
- enable qname minimization (for privacy)
- add a minimum TTL of 5m (prevent excessive resource usage and naive rebinding attacks)

## Prerequisites
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.

- [x] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/workflow.html#changelog) entry file.


Co-authored-by: Florent Daigniere <nextgens@freenetproject.org>

optional/unbound/unbound.conf

@@ -1,19 +1,20 @@
 server:
   verbosity: 1
   interface: 0.0.0.0
-  interface: ::0
+  {{ 'interface: ::0' if SUBNET6 }}
   logfile: ""
   do-ip4: yes
-  do-ip6: yes
+  do-ip6: {{ 'yes' if SUBNET6 else 'no' }}
   do-udp: yes
   do-tcp: yes
   do-daemonize: no
   access-control: {{ SUBNET }} allow
+  {{ 'access-control: {{ SUBNET6 }} allow' if SUBNET6 }}
   directory: "/etc/unbound"
   username: unbound
   auto-trust-anchor-file: trusted-key.key
   root-hints: "/etc/unbound/root.hints"
   hide-identity: yes
   hide-version: yes
-  max-udp-size: 4096
-  msg-buffer-size: 65552
+  cache-min-ttl: 300
+

towncrier/newsfragments/1992.enhancement

@@ -0,0 +1,3 @@
+Make unbound work with ipv6
+Add a cache-min-ttl of 5minutes
+Enable qname minimisation (privacy)