leapseconds: Update to the canonical place.

IERS is the source of truth for leap seconds. Their leapsecond file is
updated most quickly and is always right (unlike the IANA one which
often lags). IERS operates this public service for the express purpose
of random people downloading it. Their terms of service are compatible
with open source (we could include this in our release). Rather than
fighting with questions around this because the IANA one changed
locations or the auto update script broken, just use this.

This is in preference to the NIST ftp copy. NIST is in the process of
retiring their FTP services.

Sponsored by: Netflix
Reviewed by: philip, delphij, cy
Differential Revision: https://reviews.freebsd.org/D43752