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/smtp.c

@@ -1233,7 +1233,7 @@ static CURLcode smtp_done(struct connectdata *conn, CURLcode status,
     }
     else {
       /* Successfully sent so adjust the response timeout relative to now */
-      pp->response = Curl_tvnow();
+      pp->response = Curl_now();
 
       free(eob);
     }