Most backends don't set it nb_messages and no application used it.
No backends set nb_unread and only one application tried to use it.
This should help avoid useless fillobj() calls in some cases.
fixes#827
It is unused, and no backend implemented it — so it was wrong to have it
in the function signature.
It also does not seem very helpful, we have get_threads() if we want to
do something advanced with treads.
fixes#825
