(tar-header): New field `header-start'.

(tar-header): New field `header-start'.
(tar-header-block-tokenize): Set it when useful.
Drop "GNUtar " magic value, which even GNU Tar doesn't know about.
(tar-header-data-end): New function.
(tar-summarize-buffer): Use it.
(tar-next-line): Fix goal column for long usernames.
(tar-expunge-internal): Use header-start.
(tar-rename-entry): Handle ustar-style long names.
(tar-alter-one-field): Add optional `descriptor' argument.
(tar-subfile-save-buffer): Use it.