Add support for automatic leap-second file updates.

Add support for automatic leap-second file updates.

The working copy of leapfile resides in /var/dbntpd.leap-seconds.list.
/etc/ntp/leap-seconds (periodically updated from ftp://time.nist.gov/pub/
or ftp://tycho.usno.navy.mil/pub/ntp/) contains the master copy should
automatic leapfile updates be disabled (default).

Automatic leapfile updates are fetched from $ntp_leapfile_sources,
defaulting to https://www.ietf.org/timezones/data/leap-seconds.list,
within $ntp_leapfile_expiry_days (default 30 days) from leap-seconds
file expiry. Automatic updates can be enabled by setting
$daily_ntpd_leapfile_enable="YES" in periodic.conf. To avoid congesting
the ntp leapfile source the automatic update randomized by default but
can be disabled through daily_ntpd_avoid_congestion="NO" in
periodic.conf.

Suggested by:	des
Reviewed by:	des, roberto, dwmalone, ian, cperciva, glebius, gjb
MFC after:	1 week
X-MFC with:	r289421, r293037