bpo-38234: read_pth_file() now returns PyStatus (GH-16338)

Refactor path configuration code:

* read_pth_file() now returns PyStatus to report errors, rather than
  calling Py_FatalError().
* Move argv0_path and zip_path buffers out of PyCalculatePath
  structures.
* On Windows, _PyPathConfig.home is now preferred over PyConfig.home.

Author: vstinner