(tar-file-name-coding-system): New variable. Make

(tar-file-name-coding-system): New variable.  Make
it permanent-local.p
(tar-header-block-tokenize): Decode filename and linkname by
tar-file-name-coding-system.
(tar-header-block-checksum): Call multibyte-char-to-unibyte to get
the byte value of eight-bit chars.
(tar-summarize-buffer): Call set-buffer-multibyte with METHOD
`to'.  Delete unnecessary call of position-bytes.
(tar-mode): Set tar-file-name-coding-system.  Delete unnecessary
call of position-bytes.
(tar-extract): Simplified by calling decode-coding-region with
DESTINATION argument.  Don't toggle multibyteness of tar buffer.
(tar-copy): Don't toggle multibyteness of tar buffer.
(tar-expunge): Likewise.
(tar-clear-modification-flags): Delete unnecessary call of
position-bytes.
(tar-rename-entry): Call tar-alter-one-field with encoded new
name.
(tar-alter-one-field): Don't toggle multibyteness of tar buffer.
Convert new-data-string by string-to-multibyte before inserting
it.
(tar-subfile-save-buffer): Don't toggle multibyteness of tar
buffer.  Simplified by calling encoding-coding-region with
DESTINATION argument.
(tar-mode-write-file): Delete unnecessary call of
byte-to-position.