bpo-38353: getpath.c: allocates strings on the heap (GH-16585)

* _Py_FindEnvConfigValue() now returns a string allocated
  by PyMem_RawMalloc().
* calculate_init() now decodes VPATH macro.
* Add calculate_open_pyenv() function.
* Add substring() and joinpath2() functions.

* Fix add_exe_suffix()

And a few cleanup changes.

Author: vstinner

Include/internal/pycore_pathconfig.h

@@ -56,11 +56,10 @@ extern PyStatus _PyPathConfig_Calculate(
 extern int _PyPathConfig_ComputeSysPath0(
     const PyWideStringList *argv,
     PyObject **path0);
-extern int _Py_FindEnvConfigValue(
+extern PyStatus _Py_FindEnvConfigValue(
     FILE *env_file,
     const wchar_t *key,
-    wchar_t *value,
-    size_t value_size);
+    wchar_t **value_p);
 
 #ifdef MS_WINDOWS
 extern wchar_t* _Py_GetDLLPath(void);