Conditionally handle the crontab entry for atrun(8)

The default crontab prior to this commit assumes atrun(8) is always
present, which isn't true if MK_AT == no. Move atrun(8) execution
from /etc/crontab to /etc/cron.d/at, and base /etc/cron.d/at's installation
on MK_AT. cron(8) will detect /etc/cron.d/at's presence when the configuration
is loaded and run atrun every 5 minutes like it would prior to this commit.

SHELL and PATH are duplicated between /etc/crontab and /etc/cron.d/at
because atrun(8) executes programs, which may rely on environment
set in the current default /etc/crontab.

Noted by: bdrewery (in an internal review)
MFC after: 2 months
Relnotes: yes (may need to add environmental modifications to

		     /etc/cron.d/at)

Sponsored by: Dell EMC Isilon