Fixed regex pattern for dkim_selector #6459

Changed the regex pattern for dkim_selectors again. We now follow
the DKIMCore technical specification as stated on the website:

A selector is a string of no more than 63 lower-case alphanumeric characters (a-z or 0-9) followed by a period “.”, followed by another string of no more than 63 lower-case alphanumeric characters.

Author: Demian

interface/web/mail/form/mail_domain.tform.php

@@ -130,9 +130,9 @@
 			'default'       => 'default',
 			'value'         => 'default',
 			'width'  => '20',
-			'maxlength' => '63',
+			'maxlength' => '126',
 			'validators' => array (  0 => array (   'type' => 'REGEX',
-					'regex' => '/^(?=.*[a-z])[a-z0-9]{1,63}$/',
+					'regex' => '/^[a-z0-9]{1,63}(?:\.[a-z0-9]{1,63})?$/',
 					'errmsg'=> 'dkim_selector_error'),
 			),
 		),