title: gksu -u user2 does not ask for user2's password
body: <p>on ubuntu 10.04 i have changed the <code>Exec</code> entry in the firefox desktop shortcut to this:</p>
  
  <pre><code>Exec=/home/user1/Desktop/.unpriv_ff
  </code></pre>
  
  <p>and</p>
  
  <pre><code>$ cat /home/user1/Desktop/.unpriv_ff 
  #!/bin/bash
  xhost local: &amp;&amp; gksu -u user2 firefox
  </code></pre>
  
  <p>this allows user1 to run firefox as user2, which is safer in case of an exploit in firefox (the exploiter would only be able to read and alter user2's files and user1 would remain safe). this all works fine, however when i click the desktop icon under user1's account the prompt comes up it says:</p>
  
  <pre><code>enter your password to run application 'firefox' as user user2
  </code></pre>
  
  <p>i was expecting this to ask me to enter user2's password but instead it asks me for user1's password (and accepts it). it works fine but <strong>i was wondering if there is a way to get the prompt to ask for user2's password?</strong> as it is the functionality seems more like a request for a <code>sudo</code> password which then uses root privileges to run an application as user2.</p>