mailbox: remove mailbox_findopen - not used any more
append: get rid of the mailbox lifetime mismatch

If you do append_setup, the append struct manages the
mailbox lifetime.

If you do append_setup_mbox, the caller is responsible
for creating a locked mailbox struct and closing it again.

Simple.
mailbox: remove mailbox_ref

This abortion is no longer required, because we don't have the
append state being unbalanced any more.