(rmail-message-subject-p): Process the result

(rmail-message-subject-p): Process the result
of mail-fetch-field by rmail-summary-line-decoder.
(rmail-new-summary): Be sure to go to the Rmail buffer.  If
rmail-enable-mime is non-nil, set rmail-summary-buffer of
rmail-view-buffer to nil.
(rmail-summary-undelete): If rmail-enable-mime is non-nil, pop to
rmail-view-buffer.
(rmail-summary-scroll-msg-up): Handle rmail-view-buffer, not
rmail-buffer.
(rmail-summary-scroll-msg-down): Likewise.
(rmail-summary-beginning-of-message): Likewise.
(rmail-summary-wipe): Likewise.
(rmail-summary-toggle-header): Use save-window-excursion, not
save-excursion.  Update point in rmail-view-buffer, not in
rmail-buffer.
(rmail-summary-reply): Before calling rmail-reply, set buffer to
rmail-view-buffer, not rmail-buffer.