Prevent large amounts of UDP probes being sent consecutively.

We cannot reset udp_ping_sent to zero when we receive a valid reply to
an UDP probe, because that would cause a new one to be sent immediately
in try_udp(). Instead, add a bit to node_status_t to keep track of whether we
have a UDP probe that's waiting for a reply.

Thanks to Ronny Nilsson for spotting the source of the problem.