Merge branch '6070-bind-plugin-should-create-slave-subdirectory' into 'develop'

Resolve "bind plugin should create slave subdirectory"

Closes #6070

See merge request ispconfig/ispconfig3!1435

Author: Marius Burkard

server/plugins-available/bind_plugin.inc.php

@@ -452,7 +452,7 @@ function slave_update($event_name, $data) {
 
 		//* Ensure that the named slave directory is writable by the named user
 		$slave_record_dir = $dns_config['bind_zonefiles_dir'].'/'.$this->slave_zone_file_prefix();
-		if(!@is_dir($slave_record_dir)) mkdir($slave_record_dir, 0770);
+		if(!@is_dir($slave_record_dir)) mkdir($slave_record_dir, 0770, true);
 		chown($slave_record_dir, $dns_config['bind_user']);
 		chgrp($slave_record_dir, $dns_config['bind_group']);