bpo-32030: Add _PyMainInterpreterConfig_ReadEnv()

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Py_GetPath() and Py_Main() now call
_PyMainInterpreterConfig_ReadEnv() to share the same code to get
environment variables.

Changes:

• Add _PyMainInterpreterConfig_ReadEnv()
• Add _PyMainInterpreterConfig_Clear()
• Add _PyMem_RawWcsdup()
• _PyMainInterpreterConfig: rename pythonhome to home
• Rename _Py_ReadMainInterpreterConfig() to
  _PyMainInterpreterConfig_Read()
• Use _Py_INIT_USER_ERR(), instead of _Py_INIT_ERR(), for decoding
  errors: the user is able to fix the issue, it's not a bug in
  Python. Same change was made in _Py_INIT_NO_MEMORY().
• Remove _Py_GetPythonHomeWithConfig()

https://bugs.python.org/issue32030

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@ncoghlan: Previously, I added _Py_GetPythonHomeWithConfig() to be able to iterate slowly on the code. With this change, I was able to remove _Py_GetPythonHomeWithConfig(). Py_GetPath() now also uses internally _PyMainInterpreterConfig to get environment variables. This change is the implementation of what I described you by email.

This PR is only a small enhancement of the current code. You can still expect API changes. I tried to add program_name to _PyMainInterpreterConfig, but then I was stuck since _PyMainInterpreterConfig_ReadEnv() doesn't have access to argc and argv, whereas the code requires argv[0] to initialize program_name.

I have to think about it to see how program_name can be initialized properly.