[Spambayes] Moving closer to Gary's ideal

[Tim Peters]

FYI, there's a new display option best_cutoff_fp_weight.  The histogram
analysis code now finds the buckets that minimize

    best_cutoff_fp_weight * (# false positives) + (# false negatives)

By default it's 1 (minimize total # of misclassified msgs, favoring neither
kind of mistake).  If, e.g., you're happy to endure 100 false negatives to
save 1 false positive, set to 100.  Don't be surprised if your f-n rate
zooms, though!