unbound: maintain a consistent dnsbl cache state

In situations where a cache does not exist (mainly boot), a start/stop/restart of unbound
should make the system aware of the current dnsbl configuration in order to properly diff
on it. This includes unbound starting on boot.

src/etc/inc/plugins.inc.d/unbound.inc

@@ -379,6 +379,8 @@ function unbound_configure_do($verbose = false, $unused = '')
         }
     }
 
+    copy('/usr/local/etc/unbound.opnsense.d/dnsbl.conf', '/tmp/unbound_dnsbl.cache');
+
     mwexecf_bg('/usr/local/bin/flock -n -E 0 -o /tmp/unbound_start.lock /usr/local/opnsense/scripts/unbound/start.sh %s', [$domain]);
 
     if ($verbose) {