(%FOLDERS): Remove this global. Whether to print the folder's name is

(%FOLDERS): Remove this global.  Whether to print the folder's name is
now being subsumed into the marking unseen messages functionality
moving into filter_mail.

(store_message): Don't run mark messages unseen, and start returning
the stored message number for callers to use.

(filter_mail): Keep a hash of folders and message numbers, printing
each folder name only once and running one mark(1) command for each
folder to mark the new messages unseen.