Display a warning if attachments are allowed, while having the att. limit set to zero

[Phorum]

We regularly see questions about attachments not being allowed. This is normally because the attach permission is enabled, but the attachment limit still being at zero in the forums' config. We could prevent these questions by displaying a warning on the admin page.

Reported by: mmakaay
Imported from TRAC: http://trac.phorum.org/ticket/876