bpo-35134: Add Include/cpython/fileutils.h header file (GH-18493)

Move CPython C API from Include/fileutils.h into a new
Include/cpython/fileutils.h header file which is included by
Include/fileutils.h.

Exclude the following private symbols from the limited C API:

* _Py_error_handler
* _Py_GetErrorHandler()
* _Py_DecodeLocaleEx()
* _Py_EncodeLocaleEx()
diff --git a/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
index 9e6323f..0acf7f4 100644
--- a/PCbuild/pythoncore.vcxproj
+++ b/PCbuild/pythoncore.vcxproj
@@ -131,6 +131,7 @@
     <ClInclude Include="..\Include\cpython\ceval.h" />
     <ClInclude Include="..\Include\cpython\dictobject.h" />
     <ClInclude Include="..\Include\cpython\fileobject.h" />
+    <ClInclude Include="..\Include\cpython\fileutils.h" />
     <ClInclude Include="..\Include\cpython\import.h" />
     <ClInclude Include="..\Include\cpython\initconfig.h" />
     <ClInclude Include="..\Include\cpython\listobject.h" />