Handle the cron.d entry for MK_AT in cron conditionally

Install /etc/cron.d/at if MK_AT != no, always using it, which tries
to run a non-existent program via cron(8) every 5 minutes with the
default /etc/crontab, 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
currently set via /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