Reorganize handling of spam threshold scores and destiny mappings

[rjl]

Spam is unlike viruses/headers/attachments in that its diagnosis is score-based, not black-and-white. With that in mind, several different destinies may be applied to spam at different score thresholds:

* A = Tag threshold * B = Spam threshold * C = Quarantine threshold * D = Discard threshold

If score (X) is:

X < A : Ham, no tag, deliver A <= X < B : Ham, tag, deliver B <= X < C : Spam, tag, deliver C <= X < D : Spam, tag, quarantine D <= X : Spam, tag, discard