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

tests/unit/unit1399.c

@@ -80,7 +80,7 @@ static void expect_timer_seconds(struct Curl_easy *data, int seconds)
  * be 3 seconds. */
 UNITTEST_START
   struct Curl_easy data;
-  struct curltime now = Curl_tvnow();
+  struct curltime now = Curl_now();
 
   data.progress.t_nslookup = 0;
   data.progress.t_connect = 0;