improve python -m ipykernel help

Ok, I know I am annoying and this is likely an edge case, but when starting with `python -m` the script name is `__main__.py` ... which does not look nice :-) 

```
$ python -m ipykernel install --help
usage: __main__.py [-h] [--user] [--name NAME] [--display-name DISPLAY_NAME]
                   [--prefix PREFIX]

Install the IPython kernel spec.
```
