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

docs/examples/anyauthput.c

@@ -104,6 +104,7 @@ int main(int argc, char **argv)
     return 2;
 
 #ifdef UNDER_CE
+  /* !checksrc! disable BANNEDFUNC 1 */
   stat(file, &file_info);
 #else
   fstat(fileno(fp), &file_info);

docs/examples/fileupload.c

@@ -52,6 +52,7 @@ int main(void)
 
   /* to get the file size */
 #ifdef UNDER_CE
+  /* !checksrc! disable BANNEDFUNC 1 */
   if(stat("debugit", &file_info) != 0) {
 #else
   if(fstat(fileno(fd), &file_info) != 0) {

docs/examples/ftpupload.c

@@ -96,6 +96,7 @@ int main(void)
 
   /* to get the file size */
 #ifdef UNDER_CE
+  /* !checksrc! disable BANNEDFUNC 1 */
   if(stat(LOCAL_FILE, &file_info) != 0) {
 #else
   if(fstat(fileno(hd_src), &file_info) != 0) {

docs/examples/http2-upload.c

@@ -234,6 +234,7 @@ static int setup(struct input *i, int num, const char *upload)
   }
 
 #ifdef UNDER_CE
+  /* !checksrc! disable BANNEDFUNC 1 */
   if(stat(upload, &file_info) != 0) {
 #else
   if(fstat(fileno(i->in), &file_info) != 0) {

docs/examples/httpput.c

@@ -91,6 +91,7 @@ int main(int argc, char **argv)
 
   /* get the file size of the local file */
 #ifdef UNDER_CE
+  /* !checksrc! disable BANNEDFUNC 1 */
   if(stat(file, &file_info) != 0) {
 #else
   if(fstat(fileno(hd_src), &file_info) != 0) {