(spam-necessary-extra-headers): get the extra headers we

(spam-necessary-extra-headers): get the extra headers we
may need for spam sorting and scoring
(spam-user-format-function-S): a user format function suitable for
general use
(spam-article-sort-by-spam-status): sorting function for summary
sorting
(spam-extra-header-to-number): get a score from a header
(spam-summary-score): get a numeric score from the headers
(spam-generic-score): oops, function doc in wrong place
(spam-initialize): take symbols when it's run, and install the
extra headers that spam-necessary-extra-headers thinks we need