bpo-29240: PEP 540: Add a new UTF-8 Mode (#855)

* Add -X utf8 command line option, PYTHONUTF8 environment variable
  and a new sys.flags.utf8_mode flag.
* If the LC_CTYPE locale is "C" at startup: enable automatically the
  UTF-8 mode.
* Add _winapi.GetACP(). encodings._alias_mbcs() now calls
  _winapi.GetACP() to get the ANSI code page
* locale.getpreferredencoding() now returns 'UTF-8' in the UTF-8
  mode. As a side effect, open() now uses the UTF-8 encoding by
  default in this mode.
* Py_DecodeLocale() and Py_EncodeLocale() now use the UTF-8 encoding
  in the UTF-8 Mode.
* Update subprocess._args_from_interpreter_flags() to handle -X utf8
* Skip some tests relying on the current locale if the UTF-8 mode is
  enabled.
* Add test_utf8mode.py.
* _Py_DecodeUTF8_surrogateescape() gets a new optional parameter to
  return also the length (number of wide characters).
* pymain_get_global_config() and pymain_set_global_config() now
  always copy flag values, rather than only copying if the new value
  is greater than the old value.
diff --git a/Python/bltinmodule.c b/Python/bltinmodule.c
index 81774dc..23d7aa4 100644
--- a/Python/bltinmodule.c
+++ b/Python/bltinmodule.c
@@ -29,6 +29,9 @@
 int Py_HasFileSystemDefaultEncoding = 0;
 #endif
 const char *Py_FileSystemDefaultEncodeErrors = "surrogateescape";
+/* UTF-8 mode (PEP 540): if non-zero, use the UTF-8 encoding, and change stdin
+   and stdout error handler to "surrogateescape". */
+int Py_UTF8Mode = 0;
 
 _Py_IDENTIFIER(__builtins__);
 _Py_IDENTIFIER(__dict__);