timeout: use the correct start value as offset

Rodric provide an awesome recipe that proved libcurl didn't timeout at
the requested time - it instead often timed out at [connect time] +
[timeout time] instead of the documented and intended [timeout time]
only. This bug was due to the code using the wrong base offset when
comparing against "now". I could also take the oppurtinity to simplify
the code by properly using of the generic help function for this:
Curl_timeleft.

Reported by: Rodric Glaser
Bug: http://curl.haxx.se/bug/view.cgi?id=3061535