The second return value is (not insensibly) assumed to be an error. Instead of
returning a value there in the success case, copy the positional arguments
into the existing opts table.
This is the same as the input table (which is mutated during processing), but
if that table was created on the fly, such as by packing `...` it's convenient
if it also gets returned from the parse function.
It's not so nice to have a library that exits the entire application
from under you, so this and the error reporting belongs in util.startup.
The argparse code was originally in util.startup but moved out in
1196f1e8d178 but the error handling should have stayed.
