Usermerge support

Here's basic usermerge support:

/**
 * Implementation HOOK_usermerge_actions_supported().
 */
function listhandler_usermerge_actions_supported() {
  return array(
    'listhandler' => t('Merge listhandler message history.'),
  );
}
/**
 * Implementation of HOOK_usermerge_merge_users().
 */
function listhandler_usermerge_merge_users($user_to_delete, $user_to_keep) {
  db_query('UPDATE {listhandler} SET uid = %d WHERE uid = %d', array($user_to_keep->uid, $user_to_delete->uid));
  // Success!
  drupal_set_message(t('Successfully merged listhandler import history for %user_to_delete into %user_to_keep', array('%user_to_delete' => $user_to_delete->name, '%user_to_keep' => $user_to_keep->name)));

}