Let pyspark execute files even when IPYTHON=1

With the release of IPython 2.0 having the `PYTHONSTARTUP` environment variable and passing a file at the same time works as expected. So, we can just pass the commandline args straight to IPython.

Also remove `IPYTHON_OPTS`, as you can now just pass options to IPython directly

This puts the behavior of `IPYTHON=1` in line with the regular Python interpreter

Note that this won't work correctly when IPython version < 2.0