bpo-29240: Fix locale encodings in UTF-8 Mode

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Modify locale.localeconv(), time.tzname, os.strerror() and other
functions to ignore the UTF-8 Mode: always use the current locale
encoding.

Changes:

• Add _Py_DecodeLocaleEx() and _Py_EncodeLocaleEx(). On decoding or
  encoding error, they return the position of the error and an error
  message which are used to raise Unicode errors in
  PyUnicode_DecodeLocale() and PyUnicode_EncodeLocale().
• Replace _Py_DecodeCurrentLocale() with _Py_DecodeLocaleEx().
• PyUnicode_DecodeLocale() now uses _Py_DecodeLocaleEx() for all
  cases, especially for the strict error handler.
• Add _Py_DecodeUTF8Ex(): return more information on decoding error
  and supports the strict error handler.
• Rename _Py_EncodeUTF8_surrogateescape() to _Py_EncodeUTF8Ex().
• Replace _Py_EncodeCurrentLocale() with _Py_EncodeLocaleEx().
• Ignore the UTF-8 mode to encode/decode localeconv(), strerror()
  and time zone name.
• Remove PyUnicode_DecodeLocale(), PyUnicode_DecodeLocaleAndSize()
  and PyUnicode_EncodeLocale() now ignore the UTF-8 mode: always use
  the "current" locale.
• Remove _PyUnicode_DecodeCurrentLocale(),
  _PyUnicode_DecodeCurrentLocaleAndSize() and
  _PyUnicode_EncodeCurrentLocale().

https://bugs.python.org/issue29240

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I tested this PR using test_all_locales.py attached to https://bugs.python.org/issue29240 on Fedora 27, FreeBSD 11.0 and macOS 10.13.2. It works as expected.