- setting PYTHONIOENCODING has unintended side effects on Python 2
  instances run in a subprocess (since Python 2 has no
  `surrogateescape` error handler
- Py_SetStandardStreamEncoding enables surrogateescape for the
  current process without any side effects on subprocesses

Windows doesn't use setenv to set environment variables,
so set PYTHONIOENCODING from test_capi instead of
_testembed when running the forced_io_encoding test.

- move all required logic inside the shared library
- explicitly setting one of the coercion target locales
  now also automatically enables "surrogateescape" on
  sys.stdin and sys.stdout

Locale coercion no longer has any effect if LC_ALL is
explicitly set in the environment.
When locale coercion triggers, it sets either both
LC_CTYPE & LANG (for full locales) or only LC_CTYPE
(for partial locales).
This change also eliminated the need for a custom
test case for the locale coercion warning - instead,
the test suite is able to check for that just by
setting LC_ALL in the child process environment.

- avoid unintended side effects on Windows behaviour
- remove a single-use function that made the code harder to follow
- clarify the security considerations around ignoring -E and -I
  when checking PYTHONCOERCECLOCALE

The inline check for "Is this env var exactly zero?" is still more
self-explanatory than factoring out the helper function.

A HAVE_SETLOCALE guard was removed when adding a check for __ANDROID__,
and that may be affecting the default locale reported on Windows.

ncoghlan deleted the pep538-coerce-c-locale branch