Menu callback; logs the current user out, and redirects to the home page.
function user_logout() {
global $user;
watchdog('user', 'Session closed for %name.', array(
'%name' => $user->name,
));
module_invoke_all('user_logout', $user);
// Destroy the current session, and reset $user to the anonymous user.
session_destroy();
drupal_goto();
}