build: avoid overriding system open and stat symbols

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 #18503

Closes #18776

docs/examples/.checksrc

@@ -3,4 +3,5 @@ allowfunc fdopen
 allowfunc fopen
 allowfunc gmtime
 allowfunc localtime
+allowfunc open
 allowfunc socket