dns: make mail address of SOA records configurable

actions/admin/settings/160.nameserver.php

@@ -132,6 +132,16 @@ return array(
 					'int_min' => 3600, /* 1 hour */
 					'int_max' => 2147483647, /* integer max */
 					'save_method' => 'storeSettingField'
+				),
+				'system_soaemail' => array(
+					'label' => $lng['serversettings']['soaemail'],
+					'settinggroup' => 'system',
+					'varname' => 'soaemail',
+					'type' => 'string',
+					'string_type' => 'mail',
+					'string_emptyallowed' => true,
+					'default' => '',
+					'save_method' => 'storeSettingField'
 				)
 			)
 		)