By introducing wrappers for them in the curlx namespace:
`curlx_fopen()`, `curlx_fdopen()`, `curlx_fclose()`.
The undefine/redefine/`(function)()` methods broke on systems
implementing these functions as macros. E.g. AIX 32-bit's `fopen()`.
Also:
- rename `lib/fopen.*` to `lib/curl_fopen.*` (for `Curl_fopen()`)
to make room for the newly added `curlx/fopen.h`.
- curlx: move file-related functions from `multibyte.c` to `fopen.c`.
- tests/server: stop using the curl-specific `fopen()` implementation
on Windows. Unicode isn't used by runtests, and it isn't critical to
run tests on longs path. It can be re-enabled if this becomes
necessary, or if the wrapper receives a feature that's critical for
test servers.
Reported-by: Andrew Kirillov
Bug: https://github.com/curl/curl/issues/18510#issuecomment-3274393640
Follow-up to bf7375ecc5#18503
Follow-up to 9863599d69#18502
Follow-up to 3bb5e58c10#17827Closes#18634
