I was able to fix the test_readline failure by restoring the locale based on the environment settings with `setlocale(LC_CTYPE, "")` rather than the return value from a preceding call to `setlocale(LC_CTYPE, NULL)`.

That means we can leave the runtime coercion checks enabled on *BSD systems, and if/when any given BSD variant adds working Linux-style C.UTF-8 or OS-X-style UTF-8 locales, we'll automatically start using them.