curlx/fopen: replace open CRT functions their with _s counterparts (Windows)

- `_wopen`        -> `_wsopen_s`
- `_open`, `open` -> `_sopen_s`
- `_wfopen`       -> `_wfopen_s`
- `fopen`         -> `fopen_s`
- `_wfreopen`     -> `_wfreopen_s`
- `freopen`       -> `freopen_s`

For better error handling and for using the CRT functions recommended
via warnings suppressed by `_CRT_SECURE_NO_WARNINGS`.

Also:
- add missing `freopen_s()` prototype when building with mingw-w64 <5.
  https://sourceforge.net/p/mingw-w64/mingw-w64/ci/a5d824654cdc57f6eac1bb581b078986f3eb6856/
- tests/server: replace `open()` in the signal handler with `_sopen_s()`
  on Windows.
- tests/server: reduce scope of a checksrc exception to a single line.
- checksrc: ban replaced functions.

Refs:
https://learn.microsoft.com/cpp/c-runtime-library/reference/open-wopen
https://learn.microsoft.com/cpp/c-runtime-library/reference/sopen-s-wsopen-s
https://learn.microsoft.com/cpp/c-runtime-library/reference/freopen-wfreopen
https://learn.microsoft.com/cpp/c-runtime-library/reference/fopen-wfopen
https://learn.microsoft.com/cpp/c-runtime-library/reference/fopen-s-wfopen-s
https://learn.microsoft.com/cpp/c-runtime-library/reference/freopen-s-wfreopen-s

Closes #19643

scripts/checksrc.pl

@@ -113,6 +113,10 @@ my %banfunc = (
     "fopen" => 1,
     "freopen" => 1,
     "open" => 1,
+    "_open" => 1,
+    "_wfopen" => 1,
+    "_wfreopen" => 1,
+    "_wopen" => 1,
     "stat" => 1,
     );