Fixed #6682 -- Made shell's REPL actually execute $PYTHONSTARTUP and `~/.pythonrc.py`.

Also:

* Added a ``--no-startup`` option to disable this behavior. Previous
  logic to try to execute the code in charge of this funcionality was
  flawed (it only tried to do so if the user asked for ipython/bpython
  and they weren't found)
* Expand ``~`` in PYTHONSTARTUP value.

Thanks hekevintran at gmail dot com  for the report and initial patch.

Refs #3381.