Permissions 640 root:root doesn't allow regular user to read
/etc/{at,cron}.allow files, that changes behavior of at / crontab
commands:

* crontab command can't read /etc/cron.allow and allow any user to modify
  their crontab files.
* at command can't read /etc/at.allow and deny every user.

at / crontab files have SGID bits set, so setting correct group
on /etc/{at,cron}.allow fixes the issue.

Change-Id: I4a3fc8d8e823498d6715e26307424e3065cbd6ca