Add UDP discovery mechanism.

Add UDP discovery mechanism.

This adds a new mechanism by which tinc can determine if a node is
reachable via UDP. The new mechanism is currently redundant with the
PMTU discovery mechanism - that will be fixed in a future commit.

Conceptually, the UDP discovery mechanism works similarly to PMTU
discovery: it sends UDP probes (of minmtu size, to make sure the tunnel
is fully usable), and assumes UDP is usable if it gets replies. It
assumes UDP is broken if too much time has passed since the last reply.

The big difference with the current PMTU discovery mechanism, however,
is that UDP discovery probes are only triggered as part of the
packet TX path (through try_tx()). This is quite interesting, because
it means tinc will never send UDP pings more often than normal packets,
and most importantly, it will automatically stop sending pings as soon
as packets stop flowing, thereby nicely reducing network chatter.

Of course, there are small drawbacks in some edge cases: for example,
if a node only sends one packet every minute to another node, these
packets will only be sent over TCP, because the interval between packets
is too long for tinc to maintain the UDP tunnel. I consider this a
feature, not a bug: I believe it is appropriate to use TCP in scenarios
where traffic is negligible, so that we don't pollute the network with
pings just to maintain a UDP tunnel that's seeing negligible usage.