Release packaging & scripting improvements.

Release packaging & scripting improvements.

* Make the .xz tarball variant too, and work harder on compressing our
  files for distribution.
  + The .xz files have gained more positive feedback than any other part
    of the 4.89 release.
* Drop usercodes from tarball
  + We shouldn't be embedding own-system-specifc ownership information
    into software release tarballs.  That's for local system backups,
    not distribution.
* Script for the size/checksums
  + We include checksums in the mail; this gets the format fixed and not
    including checksums-of-signatures, etc.  I've also experimented with
    including the size, so let's script that to be portably generated.
* Better tarball signing script
  + Automatically find the signing directory (if not already in it)
  + Sign all files, properly skipping existing .asc files
  + Find the signing key from git config, if available, else error out
    (Nigel is not on the hook as the default victim now)
  + Show what we're doing as we do it

All changes made on the original `release_4_89` branch with
`RELEASE EXPERIMENT` subject tags.