Don't use a trivially predictable temporary filename and keep recreating

it again and again, practically begging the Bad Man to insert his symlink
underneath it and send us down the path to oblivion.

Noticed by:	David Lary <dlary@secureworks.net>

usr.sbin/periodic/periodic.sh

@@ -27,7 +27,7 @@ fi
 
 host=`hostname`
 export host
-tmp_output=${TMPDIR:-/tmp}/periodic.$$
+tmp_output=`mktemp ${TMPDIR:-/tmp}/periodic.XXXXXXXXXX`
 
 # Execute each executable file in the directory list.  If the x bit is not
 # set, assume the user didn't really want us to muck with it (it's a
@@ -89,9 +89,10 @@ do
                       esac
                       [ $output = TRUE ] && { cat $tmp_output; empty=FALSE; }
                     fi
-                    rm -f $tmp_output
+                    cp /dev/null $tmp_output
                 fi
             done
+            rm -f $tmp_output
         done
         if [ $empty = TRUE ]
         then