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

etc/Makefile

@@ -8,6 +8,7 @@ FILESGROUPS=	FILES
 # No need as it is empty and just causes rebuilds since this file does so much.
 UPDATE_DEPENDFILE=	no
 SUBDIR=	\
+	cron.d \
 	newsyslog.conf.d \
 	syslog.d
 

etc/cron.d/Makefile

@@ -0,0 +1,11 @@
+# $FreeBSD$
+
+.include <src.opts.mk>
+
+.if ${MK_AT} != "no"
+FILES+=		at
+.endif
+
+BINDIR=		/etc/cron.d
+
+.include <bsd.prog.mk>

etc/cron.d/at

@@ -0,0 +1,7 @@
+# $FreeBSD$
+#
+SHELL=/bin/sh
+PATH=/etc:/bin:/sbin:/usr/bin:/usr/sbin
+
+# See crontab(5) for field format.
+*/5	*	*	*	*	root	/usr/libexec/atrun

etc/crontab

@@ -7,8 +7,6 @@ PATH=/etc:/bin:/sbin:/usr/bin:/usr/sbin
 #
 #minute	hour	mday	month	wday	who	command
 #
-*/5	*	*	*	*	root	/usr/libexec/atrun
-#
 # Save some entropy so that /dev/random can re-seed on boot.
 */11	*	*	*	*	operator /usr/libexec/save-entropy
 #