…pool + ifconfig-ipv6-pool parameters to give static IP client overrides a chance to use an unallocated IP

This commit/PR depends on PR
opnsense@5f13d0a
in file src/etc/inc/plugins.inc.d/openvpn.inc which was line 999-1001
and is actual line 991-993:

            if (!empty($server['topology_subnet'])) {
                $conf .= "push \"topology subnet\"\n";
            }