• core/helper_api.php
  (helper_clear_pref_cookies): new function to clear all cookies we created to store preferences
• core/authentication_api.php
  (auth_logout): new function to perform a logout
• logout_page.php: make use of auth_logout()

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@1525 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9