Add an option to add a slight time skew to the execution of scripts to

Add an option to add a slight time skew to the execution of scripts to
smooth out load bumps, primarily for jailed environments.

Note: Modified from the original: assign the actual jitter to a local
variable instead of unnecessarily overriding one of the global variables.

Submitted-by: Michal Belczyk <belczyk@bsd.krakow.pl>
Obtained-from: FreeBSD