Fix cancellation check in ExecQueryAndProcessResults

Hi,

While looking ExecQueryAndProcessResults, I found the following code.

    /*   
     * If SIGINT is sent while the query is processing, the interrupt will be
     * consumed.  The user's intention, though, is to cancel the entire watch
     * process, so detect a sent cancellation request and exit in this case.
     */
    if (is_watch && cancel_pressed)
    {    
        ClearOrSaveAllResults();
        return 0;
    }

I guess the intention is that when the query is cancelled during \watch process,
we want to exit early before handling the error. However, we cannot detect the
cancel at this timing because currently we use PQsendQuery which is asynchronous
and does not wait  the result. We have to check cancel_pressed after PQgetResult
call. I'm also attached a patch for this, with some comments fix. 

Regards,
Yugo Nagata

-- 
Yugo NAGATA <nagata@sraoss.co.jp>