Issue #9542: Create PyUnicode_FSDecoder() function

It's a ParseTuple converter: decode bytes objects to unicode using
PyUnicode_DecodeFSDefaultAndSize(); str objects are output as-is.

 * Don't specify surrogateescape error handler in the comments nor the
   documentation, but PyUnicode_DecodeFSDefaultAndSize() and
   PyUnicode_EncodeFSDefault() because these functions use strict error handler
   for the mbcs encoding (on Windows).
 * Remove PyUnicode_FSConverter() comment in unicodeobject.c to avoid
   inconsistency with unicodeobject.h.
diff --git a/Include/unicodeobject.h b/Include/unicodeobject.h
index 7f5e8fd..cee75cc 100644
--- a/Include/unicodeobject.h
+++ b/Include/unicodeobject.h
@@ -200,6 +200,7 @@
 # define PyUnicode_FromUnicode PyUnicodeUCS2_FromUnicode
 # define PyUnicode_FromWideChar PyUnicodeUCS2_FromWideChar
 # define PyUnicode_FSConverter PyUnicodeUCS2_FSConverter
+# define PyUnicode_FSDecoder PyUnicodeUCS2_FSDecoder
 # define PyUnicode_GetDefaultEncoding PyUnicodeUCS2_GetDefaultEncoding
 # define PyUnicode_GetMax PyUnicodeUCS2_GetMax
 # define PyUnicode_GetSize PyUnicodeUCS2_GetSize
@@ -300,6 +301,7 @@
 # define PyUnicode_FromUnicode PyUnicodeUCS4_FromUnicode
 # define PyUnicode_FromWideChar PyUnicodeUCS4_FromWideChar
 # define PyUnicode_FSConverter PyUnicodeUCS4_FSConverter
+# define PyUnicode_FSDecoder PyUnicodeUCS4_FSDecoder
 # define PyUnicode_GetDefaultEncoding PyUnicodeUCS4_GetDefaultEncoding
 # define PyUnicode_GetMax PyUnicodeUCS4_GetMax
 # define PyUnicode_GetSize PyUnicodeUCS4_GetSize
@@ -1239,12 +1241,16 @@
 
 /* --- File system encoding ---------------------------------------------- */
 
-/* ParseTuple converter which converts a Unicode object into the file
-   system encoding as a bytes object, using the "surrogateescape" error
-   handler; bytes objects are output as-is. */
+/* ParseTuple converter: encode str objects to bytes using
+   PyUnicode_EncodeFSDefault(); bytes objects are output as-is. */
 
 PyAPI_FUNC(int) PyUnicode_FSConverter(PyObject*, void*);
 
+/* ParseTuple converter: decode bytes objects to unicode using
+   PyUnicode_DecodeFSDefaultAndSize(); str objects are output as-is. */
+
+PyAPI_FUNC(int) PyUnicode_FSDecoder(PyObject*, void*);
+
 /* Decode a null-terminated string using Py_FileSystemDefaultEncoding
    and the "surrogateescape" error handler.