right_main.php fix makes sure that cache is not used, when $msgs is not
registered in session. Previous code was broken when $use_mailbox_cache
is set to true, but $msgs is not in session (right_main.php switching
to non-paginated code. Depends on script load order).

move_messages.php fix updates use_mailbox_cache URL parameter. Script
does not update $msgs when messages are moved or deleted and
use_mailbox_cache can force use of outdated mailbox data.