Warn users when files override built in packages that could have caused the kernel to fail

[DonJayamanne]

We still have a large number of users running into issues resulting from creating files such as string.py, email.py, os.py, sys.py etc
I think in the past 3 weeks we had 3 users running into this issue.

This causes a lot of issues, kernel don't start or when packages get imported dynamically kernels behave weirdly.

Solution:

• Keep a hardcoded list of reserved file names (use help('modules') to get a list of all built in modules)
  This should also help Python extension, but not sure it matters as much for Python extension.

[greazer]

We've seen many users run into problems due to this issue. It affects us more than plain Python files.