Handle process INT signal.

def _sig_handler(self, signum, stack):
        '''
        Handle process INT signal.
        '''
        log_debug("Got SIGINT.")
        if signum == signal.SIGINT:
            LLNetReal.running = False
            if self._pktqueue.qsize() == 0:
                # put dummy pkt in queue to unblock a 
                # possibly stuck user thread
                self._pktqueue.put( (None,None,None) )