multi: add timer inaccuracy margin to timeout/connecttimeout

Since all systems have inaccuracy in the timeout handling it is
imperative that we add an inaccuracy margin to the general timeout and
connecttimeout handling with the multi interface. This way, when the
timeout fires we should be fairly sure that it has passed the timeout
value and will be suitably detected.

For cases where the timeout fire before the actual timeout, we would
otherwise consume the timeout action and still not run the timeout code
since the condition wasn't met.

Reported-by: He Qin
Bug: http://curl.haxx.se/bug/view.cgi?id=1298