Use buffer-swap-text to separate summary and raw data.

Use buffer-swap-text to separate summary and raw data.
(tar-header-offset): Remove.
(tar-parse-info, tar-header-offset, tar-file-name-coding-system):
Not permanent any more.
(tar-data-buffer): New var.
(tar-data-swapped-p, tar-change-major-mode-hook)
(tar-mode-kill-buffer-hook): New funs.
(tar-untar-buffer, tar-summarize-buffer, tar-mode, tar-mode-revert)
(tar-extract, tar-copy, tar-expunge-internal, tar-expunge)
(tar-clear-modification-flags, tar-alter-one-field)
(tar-subfile-save-buffer, tar-pad-to-blocksize, tar-mode-write-file):
Change accordingly.