Use defstruct and markers.

Use defstruct and markers.
(tar-setf): Remove.
(tar-header): Use defstruct.  Add `data-start' field.
(make-tar-desc, tar-desc-tokens): Remove, folded into tar-header.
(tar-desc-data-start): Remove (now called tar-header-data-start).
(tar-roundup-512): New fun.
(tar-header-block-tokenize): Receive a buffer position rather than
a string.  Handle @longLink here, be more careful about it.
Create a marker for data-start.
(tar-summarize-buffer): Don't handle @LongLink here any more.
(tar-expunge-internal, tar-subfile-save-buffer): Don't update
data-start on the following entries any more.
(tar-chown-entry, tar-chgrp-entry): Use read-number.