(rmail-view-buffer): Make buffer-local.

(rmail-view-buffer): Make buffer-local.
(rmail-select-summary, rmail-perm-variables, rmail-redecode-body)
(rmail-undelete-previous-message, rmail-mark-message)
(rmail-speedbar-buttons): Use with-current-buffer.
(rmail-mode-map): Move initialization into declaration.
(rmail-swap-buffers, rmail-unfontify-buffer-function)
(rmail-fontify-message): Use restore-buffer-modified-p.
(rmail-expunge-and-save): Don't swap buffer.
(rmail-get-header, rmail-set-header, rmail-set-attribute)
(rmail-apply-in-message): Swap save-excursion and save-restriction.