time: rename Curl_tvnow to Curl_now

... since the 'tv' stood for timeval and this function does not return a
timeval struct anymore.

Also, cleaned up the Curl_timediff*() functions to avoid typecasts and
clean up the descriptive comments.

Closes #2011

lib/asyn-thread.c

@@ -535,7 +535,7 @@ CURLcode Curl_resolver_is_resolved(struct connectdata *conn,
   }
   else {
     /* poll for name lookup done with exponential backoff up to 250ms */
-    timediff_t elapsed = Curl_timediff(Curl_tvnow(),
+    timediff_t elapsed = Curl_timediff(Curl_now(),
                                        data->progress.t_startsingle);
     if(elapsed < 0)
       elapsed = 0;