Collapse the spam-checking and filtering passes into a single pass and

adapt the diagnostic output to make sense in the new single pass.

(store_message): Don't print anything.

(preprocess): Move the message sorting to MAIN and spam processing to
filter_mail.

(filter_mail): Do spam filtering in this loop and do all status
printing here.

(%FOLDERS): This should not have been global; move it into
filter_mail.

(MAIN): Sort messages before calling filter_mail.  If any SPAM
messages were detected, scan(1) them.