timeouts: move ms timeouts to timediff_t from int and long

Now that all functions in select.[ch] take timediff_t instead
of the limited int or long, we can remove type conversions
and related preprocessor checks to silence compiler warnings.

Avoiding conversions from time_t was already done in 842f73de.

Based upon #5262
Supersedes #5214, #5220 and #5221
Follow up to #5343 and #5479
Closes #5490

lib/multi.c

@@ -1255,7 +1255,7 @@ static CURLMcode Curl_multi_wait(struct Curl_multi *multi,
          timeout */
       else if((sleep_ms < 0) && extrawait)
         sleep_ms = timeout_ms;
-      Curl_wait_ms((int)sleep_ms);
+      Curl_wait_ms(sleep_ms);
     }
   }