[PATCH v4 0/5] friendlier handling of overflows in archive-tar

This is a re-roll of the jk/big-and-future-archive-tar topic. It addresses all but one of the review comments, and I hope should be pretty polished.

The changes are:

  - the dependency on bunzip2 is dropped; instead, we just provide a
    partial object for the 64GB blob. See the first commit message for
    details.

  - the portable "head -c" replacement from t9300 has been factored out,
    and we use it in the new tests

  - symbolic constants for the giant octal numbers (with a comment
    warning that the values are set by posix)

  - the comments for tar_info() and the lazy-prereq were split so the
    two aren't mashed together

  - uses awk in tar_info() instead of "sed | cut"

  - extra simplification in the final commit, as suggested by review

  - typo and awkwardness fixes in the commit messages

The one thing that isn't fixed is the use of "141" to test for sigpipe death. That should use test_match_signal, but that topic just got re-rolled, too.

  [1/5]: t9300: factor out portable "head -c" replacement
  [2/5]: t5000: test tar files that overflow ustar headers
  [3/5]: archive-tar: write extended headers for file sizes >= 8GB
  [4/5]: archive-tar: write extended headers for far-future mtime
  [5/5]: archive-tar: drop return value

-Peff