New user: Allow @ in username

[mj72]

At www.xxx.com/members/signup.php:

Allow @ in username
- so you can use mailaddress like in every other webshop.

ftp > members > check_login.php: replace with
if ( $_REQUEST['login'] == "" or ! preg_match( "/^[A-Za-z]{1,}[A-Za-z0-9_\.\-\@]{4,}$/",
$_REQUEST['login'] ) )