Replace the leap-seconds file in r320242 from USNO -
ftp://tycho.usno.navy.mil/pub/ntp/leap-seconds.3701462400 - with a
leap-seconds file from NIST at ftp://time.nist.gov/pub/. The USNO
version of the file changes the last documented leap second update
time whereas the NIST version does not. The expiration of the USNO
version of the file is also one month short.

Requested by: ian@
Obtained from: ftp://time.nist.gov/pub/leap-seconds.3676924800
MFC after: 3 days