Replace them by `curlx_open()` and `curlx_stat()`.
To make it obvious in the source code what is being executed.
Also:
- tests/server: stop overriding `open()` for test servers.
This is critical for the call made from the signal handler.
For other calls, it's an option to use `curlx_open()`, but
doesn't look important enough to do it, following the path
taken with `fopen()`.
Follow-up to 10bac43b87#18774
Follow-up to 20142f5d06#18634
Follow-up to bf7375ecc5#18503Closes#18776
9678ff5b1b