Implement POSIX.1-2001/pax and GNU extensions for long paths and links
in `untar` and `tar`. Add a `#:format` argument to `tar` to select
among POSIX.1-2001/pax, GNU, or error encoding for long paths.
Also, add `#:skip-filtered-directory?` to `find-files`.
Less significantly, adjust `pathlist-closure` to be consistent in the
way that it includes a separator at the end of a directory path.
Also, strengthen the checking that `#:permissive?` (off by default)
performs for `untar` and `untgz` to disallow a link whose target is an
absolute path or has an up-directory element.
