Avoid type juggling vulnerability.

Password comparison should not be done with the `==` operator, but
`===`, due to type juggling.

References:

* http://phpsadness.com/sad/47
*
turbochaos.blogspot.fr/2013/08/exploiting-exotic-bugs-php-type-juggling.
html

### Test case

* Create an administrator with the password "240610708".
* Try to login to the dashboard with the password "QNKCDZO" :-)

admin/inc/auth.inc.php

@@ -1,11 +1,11 @@
 <?php
 include (dirname(__FILE__).'/pwd.inc.php');
 
-if ( isset($_COOKIE['auth']) && $_COOKIE['auth'] == $password ) {
+if ( isset($_COOKIE['auth']) && $_COOKIE['auth'] === $password ) {
     //ok, cool
 } else {
     setcookie('auth','', time()-3600);
     header('Location: login.php');
     die;
 }
-?>
+?>