docs/archives/ interoperability considerations

Action items from the recent mailing list thread Reproducing tarballs under various toolchains:

• Document in docs/archives/ that FreeBSD tar supports --owner and --group under the names --uid/--uname and --gid/--gname.
• Discuss on the site the desirability of being able to reproduce any particular tarball using a different tar implementation than the one that originally generated that tarball.
• (not directly related to the website) Send to libarchive patches implementing GNU tar compatibility flags --owner, --group, and/or send to GNU tar patches implementing BSD tar compatibility flags --uid,--uname,--gid,--gname.