Avoided use of "let" in releng/run_all_auto in favor of "expr"

releng/run_all_auto

@@ -144,7 +144,7 @@ fi
 #   echo "RET='$RET'" >/dev/tty
 
     T2=`date '+%s'`
-    let TS="${T2} - ${T1}"
+    TS=`expr ${T2} - ${T1}`
     case ${RET} in
          0)
            echo "done in ${TS} sec. ok."
@@ -181,7 +181,7 @@ fi
       if [ ${E2} -eq ${E1} ]; then
         echo " Total elapsed 0 sec." | tee -a ${CLOG}
       else
-        let ES="${E2} - ${E1}"
+        ES=`expr ${E2} - ${E1}`
         echo " Total elapsed ${ES} sec." | tee -a ${CLOG}
       fi
       #####