minimal changeset to get things working on 18.04

@joshdata squashed pull request #1398, removed some comments, and added these notes:

* The old init.d script for the management daemon is replaced with a systemd service.
* A systemd service configuration is added to configure permissions for munin on startup.
* nginx SSL settings are updated because nginx's options and defaults have changed, and we now enable http2.
* Automatic SSHFP record generation is updated to know that 22 is the default SSH daemon port, since it is no longer explicit in sshd_config.
* The dovecot-lucene package is dropped because the Mail-in-a-Box PPA where we built the package has not been updated for Ubuntu 18.04.
* The stock postgrey package is installed instead of the one from our PPA (which we no longer support), which loses the automatic whitelisting of DNSWL.org-whitelisted senders.
* Drop memcached and the status check for memcached, which we used to use with ownCloud long ago but are no longer installing.
* Other minor changes.

setup/dkim.sh

@@ -21,6 +21,11 @@ mkdir -p $STORAGE_ROOT/mail/dkim
 # Not quite sure why.
 echo "127.0.0.1" > /etc/opendkim/TrustedHosts
 
+# We need to at least create these files, since we reference them later.
+# Otherwise, opendkim startup will fail
+touch /etc/opendkim/KeyTable
+touch /etc/opendkim/SigningTable
+
 if grep -q "ExternalIgnoreList" /etc/opendkim.conf; then
 	true # already done #NODOC
 else
@@ -75,6 +80,9 @@ tools/editconf.py /etc/postfix/main.cf \
 	non_smtpd_milters=\$smtpd_milters \
 	milter_default_action=accept
 
+# We need to explicitly enable the opendmarc service, or it will not start
+hide_output systemctl enable opendmarc
+
 # Restart services.
 restart_service opendkim
 restart_service opendmarc