Replace the leap-seconds file in r347488 from USNO with a
leap-seconds file from NIST at ftp://ftp.nist.gov/pub/time.

Future updates should use the NIST version of file, available
at ftp://ftp.nist.gov/pub/time/leap-seconds.list .

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