update.php: die helpfully if getopt fails

getopt() was failing for me on a shared host with
register_argc_argv=Off and it took me a while to figure out the
problem.

* The first change makes the "don't run from a browser" feature more
  robust when register_argc_argv is disabled.
* The second change assumes cli operation and getopt failure. Since we
  need getopt to continue, we should die.
follow UNIX convention: die with nonzero exit code