periodic: replace "tty" with "test -t 0"

Apparently using tty for this purpose has been deprecated since 4.4 Lite.

Reviewed by:	cy
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D21318

usr.sbin/periodic/periodic.sh

@@ -84,7 +84,7 @@ else
 	# remove the file.
 	remove_periodic_anticongestion_file=no
 fi
-if tty > /dev/null 2>&1; then
+if [ -t 0 ]; then
 	export PERIODIC_IS_INTERACTIVE=1
 fi
 tmp_output=`mktemp ${TMPDIR:-/tmp}/periodic.XXXXXXXXXX`