build: tidy up curl-specific fstat calls and stat struct type

To avoid redefining the `fstat` system symbol, and to clarify
`struct_stat` is a curl symbol.

- introduce `curlx_fstat()` macro and use it.
- rename `struct_stat` to `curl_struct_stat`.

Also:
- tests: replace direct `curlx_win32_stat()` call with `curlx_stat()`.
- checksrc: disallow direct `_fstati64` and `fstat()` calls, except in
  examples.

Closes #20496

src/tool_doswin.c

@@ -291,7 +291,7 @@ static SANITIZEcode rename_if_reserved_dos(char ** const sanitized,
    * to rename such files. */
   char *p, *base, *buffer;
 #ifdef MSDOS
-  struct_stat st_buf;
+  curl_struct_stat st_buf;
 #endif
   size_t len, bufsize;
 

src/tool_filetime.c

@@ -68,7 +68,7 @@ int getfiletime(const char *filename, curl_off_t *stamp)
           GetLastError());
   }
 #else
-  struct_stat statbuf;
+  curl_struct_stat statbuf;
   if(curlx_stat(filename, &statbuf) != -1) {
     *stamp = (curl_off_t)statbuf.st_mtime;
     rc = 0;

src/tool_formparse.c

@@ -122,13 +122,13 @@ static struct tool_mime *tool_mime_new_filedata(struct tool_mime *parent,
     char *data = NULL;
     curl_off_t size;
     curl_off_t origin;
-    struct_stat sbuf;
+    curl_struct_stat sbuf;
 
     CURLX_SET_BINMODE(stdin);
     origin = ftell(stdin);
     /* If stdin is a regular file, do not buffer data but read it
        when needed. */
-    if(fd >= 0 && origin >= 0 && !fstat(fd, &sbuf) &&
+    if(fd >= 0 && origin >= 0 && !curlx_fstat(fd, &sbuf) &&
 #ifdef __VMS
        sbuf.st_fab_rfm != FAB$C_VAR && sbuf.st_fab_rfm != FAB$C_VFC &&
 #endif

src/tool_getparam.c

@@ -2209,7 +2209,7 @@ static ParameterError existingfile(char **store,
                                    const struct LongShort *a,
                                    const char *filename)
 {
-  struct_stat info;
+  curl_struct_stat info;
   if(curlx_stat(filename, &info)) {
     errorf("The file '%s' provided to --%s does not exist",
            filename, a->lname);

src/tool_operate.c

@@ -146,7 +146,7 @@ static bool is_pkcs11_uri(const char *string)
  *
  */
 static curl_off_t vms_realfilesize(const char *name,
-                                   const struct_stat *stat_buf)
+                                   const curl_struct_stat *stat_buf)
 {
   char buffer[8192];
   curl_off_t count;
@@ -176,7 +176,8 @@ static curl_off_t vms_realfilesize(const char *name,
  *  if not to call a routine to get the correct size.
  *
  */
-static curl_off_t VmsSpecialSize(const char *name, const struct_stat *stat_buf)
+static curl_off_t VmsSpecialSize(const char *name,
+                                 const curl_struct_stat *stat_buf)
 {
   switch(stat_buf->st_fab_rfm) {
   case FAB$C_VAR:
@@ -247,7 +248,7 @@ static struct per_transfer *del_per_transfer(struct per_transfer *per)
 static CURLcode pre_transfer(struct per_transfer *per)
 {
   curl_off_t uploadfilesize = -1;
-  struct_stat fileinfo;
+  curl_struct_stat fileinfo;
   CURLcode result = CURLE_OK;
 
   if(per->uploadfile && !stdin_upload(per->uploadfile)) {
@@ -284,7 +285,7 @@ static CURLcode pre_transfer(struct per_transfer *per)
     if(per->infd == -1)
 #else
       per->infd = curlx_open(per->uploadfile, O_RDONLY | CURL_O_BINARY);
-    if((per->infd == -1) || fstat(per->infd, &fileinfo))
+    if((per->infd == -1) || curlx_fstat(per->infd, &fileinfo))
 #endif
     {
       helpf("cannot open '%s'", per->uploadfile);
@@ -547,11 +548,11 @@ static CURLcode retrycheck(struct OperationConfig *config,
     }
 
     if(truncate && outs->bytes && outs->filename && outs->stream) {
-      struct_stat fileinfo;
+      curl_struct_stat fileinfo;
 
       /* The output can be a named pipe or a character device etc that
          cannot be truncated. Only truncate regular files. */
-      if(!fstat(fileno(outs->stream), &fileinfo) &&
+      if(!curlx_fstat(fileno(outs->stream), &fileinfo) &&
          S_ISREG(fileinfo.st_mode)) {
         int rc;
         /* We have written data to an output file, we truncate file */
@@ -717,7 +718,7 @@ static CURLcode post_per_transfer(struct per_transfer *per,
       errorf("curl: (%d) Failed writing body", result);
     }
     if(result && config->rm_partial) {
-      struct_stat st;
+      curl_struct_stat st;
       if(!curlx_stat(outs->filename, &st) && S_ISREG(st.st_mode)) {
         if(!unlink(outs->filename))
           notef("Removed output file: %s", outs->filename);
@@ -1039,7 +1040,7 @@ static CURLcode setup_outfile(struct OperationConfig *config,
   }
 
   if(config->skip_existing) {
-    struct_stat fileinfo;
+    curl_struct_stat fileinfo;
     if(!curlx_stat(per->outfile, &fileinfo)) {
       /* file is present */
       notef("skips transfer, \"%s\" exists locally", per->outfile);
@@ -1051,7 +1052,7 @@ static CURLcode setup_outfile(struct OperationConfig *config,
   if(config->resume_from_current) {
     /* We are told to continue from where we are now. Get the size
        of the file as it is now and open it for append instead */
-    struct_stat fileinfo;
+    curl_struct_stat fileinfo;
     /* VMS -- Danger, the filesize is only valid for stream files */
     if(curlx_stat(per->outfile, &fileinfo) == 0)
       /* set offset to current file size: */