cli.php can only be run from command line
Well, I seem to have resolved it by replacing cli.php with cron.php in the command line.  Of course it didn't do anything when I ran the command from the terminal, but I plugged it into the cron jobs list and it seems to be executing the jobs now.  I am just lucky I spotted the "cron.php" file in the bin folder, and took a chance on it being what I needed.  

My sympathies to any novice like me who has to set up Cron Jobs for CiviCRM -- the instructions may make sense to a Linux expert or php programmer, but to complete this simple and essential task, it has taken me months.  

Thanks for your help, folks!