Change rmail-spam-filter- or spam-filter-

Change rmail-spam-filter- or spam-filter-
or rmail-spam- to rsf- in all function and variable names.
(rsf-min-region-to-spam-list): New variable.
(rsf-bbdb-auto-delete-spam-entries): Renamed from
rmail-bbdb-auto-delete-spam-bbdb-entries.  The cc: field is
scanned together with the recipients field for spam testing; Don't
delete spam message if rmail-delete-after-output is non-nil;
(check-field) New function, extracted from code in
rmail-spam-filter to ease addition of header fields like content-type:.
(message-content-type) New variable.  The content-type: field was
added also in defcustom of rsf-definitions-alist;
(rmail-spam-filter): Replace repeated test code for header fields
by calls to check-field; change the call to
rmail-output-to-rmail-file such that rmail-current-message stays
the same to avoid wrong deletion of unseen flags.
(rmail-use-spam-filter): Add autoload cookie.