Fix a know bug in `multiprocessing`'s forking module's `Popen` under python 2.6. Refs #4008.

We created `salt.utils.multiprocess` which, when under python 2.6, defines the `Popen` class to be used with `multiprocessing.Process`.
The fix was taken from python's source code repository.
This should only be necessary while salt supports python 2.6.