[core] add config data type "encrypt"

ex. (in xoops_version.php)
```
$modversion['config'] = array(
    array(
        'name'        => 'password',
        'title'       => '_MI_XXX_PASS',
        'description' => '_MI_XXX_PASS_DESC',
        'formtype'    => 'password',
        'valuetype'   => 'encrypt',
        'default'     => '',
        'options'     => array(),
    ) ,
);
```
In this case, the data 'password', which is stored in the database is
encrypted.