Consolidate random sleeps in periodic scripts

[asomers]

Summary

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

[asomers]

Two things to note:

1. I have a change ready to go for sysutils/bsdstats. I haven't done ports-mgmt/pkg yet
2. I decided not to bump FreeBSD_version, because only shell scripts care about this change, and FreeBSD_version is hard to check from shell scripts. It's easier just to check for the definition of anticongestion_sleeptime

[cy]

Looks good.

[asomers]

Fix anticongestion's single-sleep feature for the security runs, which run
recursively within the daily, weekly, and monthly runs.