bpo-38353, bpo-38429: Fix typos introduced by commit
c02b41b1fb115c87693530ea6a480b2e15460424 in
calculate_argv0_path_framework() of getpath.c.

Calling _Py_wfopen() is enough to check if filename is an existing
file or not. There is no need to check first isfile().

* _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.

* PyCalculatePath: add "_macro" to the 3 fields storing macro values
* Add dynamically allocated strings to PyCalculatePath

* ismodule() no longer modify the input string.
* copy_absolute(), joinpath(): rename parameters and local variables.

Following symbolic links is now limited to 40 attempts, just to
prevent loops.

Add subfunctions:

* Add resolve_symlinks()
* Add calculate_argv0_path_framework()
* Add calculate_which()
* Add calculate_program_macos()

Fix also _Py_wreadlink(): readlink() result type is Py_ssize_t, not
int.

calculate_argv0_path() must read argv0_path link, not read
program_full_path link.

* search_for_prefix() directly calls reduce() if found is greater
  than 0.
* Add calculate_pybuilddir() subfunction.
* search_for_prefix(): add path string buffer for readability.
* Fix some error handling code paths: release resources on error.
* calculate_read_pyenv(): rename tmpbuffer to filename.
* test.pythoninfo now also logs windows.dll_path

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.