Re: Thread-safe nl_langinfo() and localeconv()

Here is a slightly better version of patch 0003. I removed some
unnecessary refactoring, making the patch smaller.

FTR I wrote a small program[1] for CI to test the assumptions about
Windows in 0001. I printed out the addresses of the objects, to
confirm that different threads were looking at different objects once
the thread local mode was activated, and also assert that the struct
contents were as expected while 8 threads switched locales in a tight
loop, and the output[2] looked OK to me.

[1] https://github.com/macdice/hello-windows/blob/793eb2fe3e6738c200781f681a22a7e6358f39e5/test.c
[2] https://cirrus-ci.com/task/4650412253380608