(tar-header-block-tokenize): Decode codes of file

(tar-header-block-tokenize): Decode codes of file
and link names if necessary.
(tar-header-block-summarize): Handle the case that file or link
names are multibyte.
(tar-summarize-buffer): At first set the current buffer unibyte,
then if there are multibyte file names, change it to multibyte.
(tar-mode): Get char position from tar-header-offset.
(tar-extract): Avoid multibyte<->unibyte conversion in
insert-buffer-subsring by setting both buffers unibyte
temporarily.
(tar-copy): Set the buffer unibyte while doing a work.  Write
without code conversion.
(tar-expunge): Set the buffer unibyte while doing a work.
(tar-alter-one-field): Likewise.
(tar-clear-modification-flags): Compare byte position with
tar-header-offset.
(tar-subfile-save-buffer): Avoid multibyte<->unibyte conversion in
insert-buffer-subsring by setting both buffers unibyte
temporarily.  Pay attention to multibyteness while updating the
descriptor-line.
(tar-mode-write-file): Write without code conversion.