There's an alternative to trying to force a different encoding for the
standard streams when the OS claims ASCII as the OS encoding: we can
default to surrogateescape as the error handler, on the assumption that
whatever the *real* OS encoding is, it definitely isn't ASCII.

That means we'll still complain about displaying improperly encoded data
when the OS suggests a plausible encoding, but we won't fail entirely just
because someone enabled (deliberately or accidentally) the POSIX locale.