mailbox: Lock destination when copying or moving
When either moving messages from one mailbox to another, or copying them, we need to lock both mailboxes, not just the source mailbox.
* libbalsa/mailbox.c
(libbalsa_mailbox_real_messages_copy): remove guards;
(messages_copy_locked): helper;
(libbalsa_mailbox_messages_copy): guard and lock destination, and use helper;
(libbalsa_mailbox_messages_move): ditto.