MFC r324681, r324738

Anticongestion refinements for ntpd rc script. This checks if ntp
leapfile needs fetching before entering into the anticongestion sleep.

Unfortunately some ports still use their own sleeps so, this commit
doesn't address the complete problem which is compounded by every
port that uses its own anticongestion mechanism.

Discussed with: asomers, feld

MFC r316342, r316358

r316342:
Consolidate random sleeps in periodic scripts

Multiple periodic scripts sleep for a random amount of time in order to
mitigate the thundering herd problem. This is bad, because the sum of
multiple uniformly distributed random variables approaches a normal
distribution, so the problem isn't mitigated as effectively as it would be
with a single sleep.

This change creates a single configurable anticongestion sleep. periodic
will only sleep if at least one script requires it, and it will never sleep
more than once per invocation. It also won't sleep if periodic was run
interactively, fixing an unrelated longstanding bug.

PR: 217055
PR: 210188
Reviewed by: cy
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D10211

r316358:
Fix man page typo from r316342

Reported by: rgrimes
MFC after: 20 days
X-MFC-With: 316342

MFC r304162

Decrease the anti-congestion sleep in 480.leapfile-ntpd to 1 hour

24 hours is too long. Periodic scripts are executed serially, so when
combined with the sleep in 410.pkg-audit periodic could actually take more
than 24 hours and block the next invocation.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Don't rely on $ntpd_enable to periodically fetch the latest
leapfile.

Suggested by: cperciva
MFC after: 1 week

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