curl: make the URL indexes 64 bit

Otherwise we could misbehave already at 2 billion URLs and we can't have
that. A few of the counters are already correctly using the right type.

Closes #18096

src/tool_operate.h

@@ -42,7 +42,7 @@ struct per_transfer {
   struct curltime start; /* start of this transfer */
   struct curltime retrystart;
   char *url;
-  unsigned int urlnum; /* the index of the given URL */
+  curl_off_t urlnum; /* the index of the given URL */
   char *outfile;
   int infd;
   struct ProgressData progressbar;