(tar-header-block-tokenize): Presume less, check more.

(tar-header-block-tokenize): Presume less, check more.
(tar-summarize-buffer): Don't silently skip incomplete headers.
(tar-mode): Revert to fundamental-mode in case of malformed tar data.
(tar-extract): Try to make sure set-auto-mode doesn't mistakenly
treat a tar file member as being a tar file itself, just because
its own filename includes the parent tar file's.