Roll back signout.php changes and move the fix to auth.php, since checking $_SESSION...

Roll back signout.php changes and move the fix to auth.php, since checking $_SESSION at the top of signout.php *before* the session was even started meant that the user was always considered to be logged out, even before they really were.  This broke most all of the plugins that hooked into signout.php

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@9055 7612ce4b-ef26-0410-bec9-ea0150e637f0