interp: panic on set builtin with the end of options flag

[dhanusaputra]

close #816

[mvdan]

Thanks for the patch! Unfortunately this is not quite right, as per help set:

      -   Assign any remaining arguments to the positional parameters.
          The -x and -v options are turned off.

That is, your added test should succeed with:

$ set - foobar
$ echo "$@"
foobar