Support Posix-standard pax extended header in tar files

Support Posix-standard pax extended header in tar files

* lisp/tar-mode.el (pax-extended-attribute-record-regexp)
(tar-attr-vector): New variables.
(pax-gid-index, pax-gname-index, pax-linkpath-index)
(pax-mtime-index, pax-path-index, pax-size-index, pax-uid-index)
(pax-uname-index): New constants.
(pax-header-gid, pax-header-gname, pax-header-linkpath)
(pax-header-mtime, pax-header-path, pax-header-size)
(pax-header-uid, pax-header-uname): New accessors to pax header.
(pax-decode-string, tar-parse-pax-extended-header): New functions.
(tar-header-block-tokenize): Recognize and handle Posix-standard
pax extended header, and use its attributes instead of those in
the standard tar header.  (Bug#64686)