* bpo-34485: stdout uses surrogateescape on POSIX locale (GH-8986)

Standard streams like sys.stdout now use the "surrogateescape" error
handler, instead of "strict", on the POSIX locale (when the C locale is not
coerced and the UTF-8 Mode is disabled).

Add tests on sys.stdout.errors with LC_ALL=POSIX.

Fix the error handler of standard streams like sys.stdout:
PYTHONIOENCODING=":" is now ignored instead of setting the error handler to
"strict".

(cherry picked from commit 315877dc)