* spam.el (spam-use-gmane-xref): new backend

* spam.el (spam-use-gmane-xref): new backend
(spam-gmane-xref-spam-group): variable to control the name of the
Gmane spam group
(spam-blackhole-servers, spam-blackhole-good-server-regex)
(spam-regex-headers-spam, spam-regex-headers-ham)
(spam-regex-body-spam, spam-regex-body-ham): clarified docs
(spam-list-of-checks): added spam-use-gmane-xref to list of
backends and checks
(spam-check-gmane-xref): function for spam-use-gmane-xref

* gnus.el (spam-autodetect-methods): add spam-use-gmane-xref as
an autodetect method