Set ``__file__`` while running a startup file.  Like Python, IDLE optionally
runs one startup file in the Shell window before presenting the first interactive
input prompt.  For IDLE, ``-s`` runs a file named in environmental variable
 :envvar:`IDLESTARTUP` or  :envvar:`PYTHONSTARTUP`; ``-r file`` runs
``file``.  Python sets ``__file__`` to the startup file name before running the
file and unsets it before the first prompt.  IDLE now does the same when run
normally, without the ``-n`` option.
