(rmail-header-attribute-header)

(rmail-header-attribute-header)
(rmail-header-keyword-header, rmail-header-get-limit): Doc.
(rmail-header-add-header): Add inhibit-point-motion-hooks and
buffer-undo-list bindings from rmail-header-set-header.  I don't
know whether they are necessary, however.
(rmail-header-persist-attributes, rmail-header-set-header):
Deleted.  rmail-header-add-header does what
rmail-header-set-header did, except throw an error if the header
was not found.