Change `setlocale()` to use the `uselocale()` C function if available on POSIX too

Hello everyone, We recently noticed that the setlocale function currently uses setlocale in its C implementation as well [https://github.com/php/frankenphp/pull/1941], which is a) not multithreading safe to use and b) when it doesn't crash, leads to overriding the current locale for the entire process. For NTS builds with php-fpm this is not an issue, but for FrankenPHP and mod_php, this leads to unexpected behaviour described here: https://github.com/php/frankenphp/pull/1941#issuecomment-3457137937 On Windows builds, since PHP 7, it is already per-thread, but on POSIX systems it is not. I would therefore like to get your reactions on whether it would make sense to unify the behaviour between POSIX systems and Windows and make the setlocale() function thread-safe and per-thread, rather than per-process. In case my proposal sees positive reception, I'd like to go ahead and create an RFC. I will also implement the changes in php-src, I've already done so as a proof of concept. Thank you for your time, Marc Henderkes