[3.6] bpo-32555: Fix locale encodings (#5193)

[3.6] bpo-32555: Fix locale encodings (#5193)

On FreeBSD and Solaris, os.strerror() now always decode the byte
string from the current locale encoding, rather than using
ASCII/surrogateescape in some cases.

Changes:

* Add _Py_DecodeLocaleEx() and _Py_EncodeLocaleEx() which has an
  additional current_locale parameter.
* PyUnicode_DecodeLocale(), PyUnicode_DecodeLocaleAndSize() and
* PyUnicode_EncodeLocale() now always use the current locale
* encoding, instead of using Py_DecodeLocale()/Py_EncodeLocale().
* Document encoding in Py_DecodeLocale() and Py_EncodeLocale()
  documentations.
* Add USE_FORCE_ASCII define to not define
  decode_ascii_surrogateescape() on Android.