stop the link

def stop_ppp_link(self):
        '''stop the link'''
        if self.ppp_fd == -1:
            return
        try:
            self.mpself.select_extra.pop(self.ppp_fd)
            os.close(self.ppp_fd)
            os.waitpid(self.pid, 0)
        except Exception:
            pass
        self.pid = -1
        self.ppp_fd = -1
        print("stopped ppp link")