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

scripts/checksrc.pl

@@ -91,6 +91,8 @@ my %banfunc = (
     "fclose" => 1,
     "fdopen" => 1,
     "fopen" => 1,
+    "open" => 1,
+    "stat" => 1,
     );
 
 my %warnings_extended = (