freebsd-update: mark "cron" as fetched as "fetch" itself

The change in 33bd05c318 was incomplete
because it did not mark "cron" as ISFETCHED=1 although it performs the
same operations as "install", but less output and does not perform a
hard exit. Mark result as such and make "install" know that updates have
been fetched.

PR:		277699
Approved by:	jrm (mentor), emaste, cperciva
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D44356

usr.sbin/freebsd-update/freebsd-update.sh

@@ -3457,6 +3457,7 @@ cmd_cron () {
 	    [ ${VERBOSELEVEL} = "debug" ]; then
 		mail -s "`hostname` security updates" ${MAILTO} < ${TMPFILE}
 	fi
+	ISFETCHED=1
 
 	rm ${TMPFILE}
 }