(rmail-automatic-folder-directives): Doc fix.

(rmail-automatic-folder-directives): Doc fix.
(rmail-current-message, rmail-total-messages)
(rmail-message-vector, rmail-deleted-vector): Add doc strings.
(rmail-duplicate-message): Doc fix.
(rmail-get-header-1, rmail-set-header-1, rmail-set-attribute-1):
New functions.
(rmail-get-header, rmail-set-header, rmail-set-attribute):
Use rmail-apply-in-message.
(rmail-message-attr-p): Use rmail-get-header, hence no longer requires
unswapped-ness.
(rmail-get-attr-names): Check for missing or corrupt attribute headers.
(rmail-auto-file): Set the filed attribute, rather than explicitly not
doing so.  (Bug#2231)