bpo-34527: POSIX locale enables the UTF-8 Mode (GH-8972)

* The UTF-8 Mode is now also enabled by the "POSIX" locale, not only
  by the "C" locale.
* On FreeBSD, Py_DecodeLocale() and Py_EncodeLocale() now also forces
  the ASCII encoding if the LC_CTYPE locale is "POSIX", not only if
  the LC_CTYPE locale is "C".
* test_utf8_mode.test_cmd_line() checks also that the command line
  arguments are decoded from UTF-8 when the the UTF-8 Mode is enabled
  with POSIX locale or C locale.