pep540.patch: first draft

Changes:

* Add sys.flags.utf8mode
* Add -X utf8 command line option
* Add PYTHONUTF8 environment variable
* sys.stdin, sys.stdout and sys.stderr encoding and errors are modified in UTF-8 mode
* open() default encoding and errors is modified in the UTF-8 mode
* Add Lib/test/test_utf8mode.py
* Skip a few tests relying on the locale encoding if the UTF-8 mode is enabled
* Document changes

Allowed options:

* Disable UTF-8 mode: -X utf8=0 or PYTHONUTF8=0
* Enable UTF-8 mode: -X utf8=1 or PYTHONUTF8=1
* Enable UTf-8 Strict mode: -X utf8=strict or PYTHONUTF8=strict
* Other -X utf8 and PYTHONUTF8 values cause a fatal error

Prioririties (highest to lowest):

* open() encoding and errors arguments
* PYTHONIOENCODING
* UTF-8 mode
* os.device_encoding()
* locale encoding

TODO:

* re-encode sys.argv from the local encoding to UTF-8 in Py_Main() when the UTF-8 mode is enabled
* support strict mode in Py_DecodeLocale() and Py_EncodeLocale()