periodic: switch lockf to silent operation.

This fixes duplicate mails (one from cron, one from periodic)
when a periodic run is not finished bfore the next one starts.

The man page states that the intended use case is cron, and
the error handling of the lockf invocation handles this case
explicitely, as such no error message for the "interactive"
use was considered.

usr.sbin/periodic/periodic.sh

@@ -53,7 +53,7 @@ if [ $1 != "LOCKED" ]; then
     ret=0
     for arg; do
         lockfile=/var/run/periodic.${arg##*/}.lock
-        lockf -t 0 "${lockfile}" /bin/sh $0 LOCKED "$arg"
+        lockf -s -t 0 "${lockfile}" /bin/sh $0 LOCKED "$arg"
         case $? in
         0) ;;
         73) #EX_CANTCREATE