bpo-38234: Add test_init_setpath_config() to test_embed (GH-16402)

* Add test_embed.test_init_setpath_config(): test Py_SetPath()
  with PyConfig.
* test_init_setpath() and test_init_setpythonhome() no longer call
  Py_SetProgramName(), but use the default program name.
* _PyPathConfig: isolated, site_import  and base_executable
  fields are now only available on Windows.
* If executable is set explicitly in the configuration, ignore
  calculated base_executable: _PyConfig_InitPathConfig() copies
  executable to base_executable.
* Complete path config documentation.