Import package from pkgsrc-wip: vixie-cron.

Import package from pkgsrc-wip: vixie-cron.
(Note this has some differences from NetBSD's cron, such
as by default users can't have own crontabs unless allowed.)

Vixie Cron is a popular implementation of cron for running scheduled
commands for the system and regular users.

This is a version of 'cron' that is known to run on most systems.  It is
functionally based on the SysV cron, which means that each user can have
their own crontab file (all crontab files are stored in a read-protected
directory).

A message is logged each time a command is executed; also, the files
"allow" and "deny" can be used to control access to the "crontab" command
(which installs crontabs).