relax SVN archive

Fix for sr #3117, (https://gna.org/support/?3117) - Functionality to inspect interactively after running script - The equivalence to python -i

This is to allow the -p --prompt option to be given without a script. relax should support this so that a user doesn't get too confused when
trying to start in prompt mode with the --prompt flag and support the --prompt argument without a script being supplied.