> It's actually "adding" the current directory

It's the script directory, which gets added on all platforms when PySys_SetArgv is called with Py_IsolatedFlag == 0. In this case we're running "__main__.py" from a directory or zip file, and we don't want its parent directory in sys.path.

> The behavior on Windows is correct 

Here's what I see on a current build of 3.6:

    C:\Temp>python_d -i main361
    >>> import sys
    >>> sys.version
    '3.6.0+ (default, Mar  8 2017, 06:51:22) [MSC v.1900 64 bit (AMD64)]'
    >>> sys.path[:2]
    ['main361', 'C:\\Temp']

C:\Temp doesn't belong in sys.path in this case. When we're not in isolated mode, RunMainFromImporter should set the __main__.py import source as index 0 rather than insert it.