(spam-cache-lookups): set to t by default

(spam-cache-lookups): set to t by default
(spam-find-spam): don't try to guess spam-cache-lookups
(spam-enter-whitelist, spam-enter-blacklist): also clear the
spam-caches entry
(spam-filelist-build-cache, spam-filelist-check-cache): better
caching of whitelist/blacklist entries
(spam-check-whitelist, spam-check-blacklist): invoke
spam-from-listed-p with a type, not a cache variable
(spam-from-listed-p): wrap around spam-filelist-check-cache