[tz] new NIST leap-seconds.list

[Chris Woodbury]

Thanks to Dr. Levine, NIST has a newer version of leap-seconds.list with
an expiry of 28 Dec 2019.  Hurrah!!  (Note: I re-named it so I could see
the different versions.)

$ FIPS180 -f NIST_leap-seconds.3676924800.2019-01-31 -v -h

        Generated SHA digest matches file digest

            File name: NIST_leap-seconds.3676924800.2019-01-3
Original file name: leap-seconds.3676924800

     Last modified: 31 Jan 2019 08:02:47 UTC

           Updated: 08 Jul 2016 00:00:00 UTC
           Expires: 28 Dec 2019 00:00:00 UTC

#h      83c68138 d3650221 07dbbbcd 11fcc859 ced1106a

$