* spam.el: added some gnus-registry autoloads

* spam.el: added some gnus-registry autoloads
(spam-split-symbolic-return): makes spam-split return 'spam
instead of the value of spam-split-group when spam is detected
(spam-split-symbolic-return-positive): makes spam-split return
'ham instead of nil when ham is detected
(spam-autodetect-recheck-messages): tells spam.el whether it
should recheck all messages in a group, or only the unseen ones
(spam-split-last-successful-check): spam-split will set this to
the last successful check; this was seen as a cleaner approach
than returning a cell like '(spam spam-use-bogofilter)
(spam-list-of-checks): documentation appended
(spam-split): accomodate the spam-split-symbolic-return and
spam-split-symbolic-return-positive variables
(spam-find-spam): new function called when the summary is built
(spam-log-registered-p): checks if a ham or spam registration has
already been done for an article
(spam-check-regex-headers, spam-check-blackholes, spam-check-BBDB)
(spam-check-ifile, spam-check-stat, spam-check-whitelist)
(spam-check-blacklist, spam-check-bogofilter-headers)
(spam-check-spamoracle): respect the spam-split-symbolic-return
and spam-split-symbolic-return-positive variables
(spam-initialize): add spam-find-spam to gnus-summary-prepare-hook
(spam-unload-hook): remove spam-find-spam from
gnus-summary-prepare-hook

* gnus.el (spam-autodetect, spam-autodetect-methods): new
configuration items for spam autodetection