tidy-up: use uppercase TRUE/FALSE where missing

Keep it only in external API calls and C++ code.

Also:
- curlx/fopen: replace with `!!`.

Spotted by GitHub Code Quality in cf-socket.c.

Closes #21925

src/tool_writeout_json.c

@@ -105,7 +105,7 @@ void ourWriteOutJSON(FILE *stream, const struct writeoutvar mappings[],
 
   for(i = 0; i < nentries; i++) {
     if(mappings[i].writefunc &&
-       mappings[i].writefunc(stream, &mappings[i], per, per_result, true))
+       mappings[i].writefunc(stream, &mappings[i], per, per_result, TRUE))
       fputs(",", stream);
   }