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

src/tool_filetime.c

@@ -75,7 +75,7 @@ int getfiletime(const char *filename, curl_off_t *stamp)
   }
 #else
   struct_stat statbuf;
-  if(stat(filename, &statbuf) != -1) {
+  if(curlx_stat(filename, &statbuf) != -1) {
     *stamp = (curl_off_t)statbuf.st_mtime;
     rc = 0;
   }