multi: during ratelimit multi_getsock should return no sockets

... as there is nothing to wait for then, it just waits. Otherwise, this
causes much more CPU work and updates than necessary during ratelimit
periods.

Ref: https://curl.se/mail/lib-2023-11/0056.html
Closes #12430

lib/multi.c

@@ -1064,6 +1064,10 @@ static void multi_getsock(struct Curl_easy *data,
   case MSTATE_PERFORMING:
     Curl_pollset_add_socks(data, ps, Curl_single_getsock);
     break;
+
+  case MSTATE_RATELIMITING:
+    /* nothing to wait for */
+    return;
   }
 
   /* Let connection filters add/remove as needed */

lib/progress.c

@@ -304,7 +304,7 @@ timediff_t Curl_pgrsLimitWaitTime(curl_off_t cursize,
    * 'actual' is the time in milliseconds it took to actually download the
    * last 'size' bytes.
    */
-  actual = Curl_timediff(now, start);
+  actual = Curl_timediff_ceil(now, start);
   if(actual < minimum) {
     /* if it downloaded the data faster than the limit, make it wait the
        difference */