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