[ticket/13055] Fix string profile fields validation to support unicode

Add unicode support to the string type based profile fields.

<a href="https://tracker.phpbb.com/browse/PHPBB3-13055">PHPBB3-13055</a>.
Alphanumeric only should not allow non-ascii characters as it should only contain latin letters and arabic numbers (a-z0-9 + _) per the definition of alphanumeric. What you added should be someting like letters and numbers only.