Add UDP_INFO protocol message.

Add UDP_INFO protocol message.

In this commit, nodes use UDP_INFO messages to provide UDP address
information. The basic principle is that the node that receives packets
sends UDP_INFO messages to the node that's sending the packets. The
message originally contains no address information, and is (hopefully)
updated with relevant address information as it gets relayed through the
metagraph - specifically, each intermediate node will update the message
with its best guess as to what the address is while forwarding it.

When a node receives an UDP_INFO message, and it doesn't have a
confirmed UDP tunnel with the originator node, it will update its
records with the new address for that node, so that it always has the
best possible guess as to how to reach that node. This applies to the
destination node of course, but also to any intermediate nodes, because
there's no reason they should pass on the free intel, and because it
results in nice behavior in the presence of relay chains (multiple nodes
in a path all trying to reach the same destination).

If, on the other hand, the node does have a confirmed UDP tunnel, it
will ignore the address information contained in the message.

In all cases, if the node that receives the message is not the
destination node specified in the message, it will forward the message
but not before overriding the address information with the one from its
own records. If the node has a confirmed UDP tunnel, that means the
message is updated with the address of the confirmed tunnel; if not,
the message simply reflects the records of the intermediate node, which
just happen to be the contents of the UDP_INFO message it just got, so
it's simply forwarded with no modification.

This is similar to the way ANS_KEY messages are currently
overloaded to provide UDP address information, with two differences:

 - UDP_INFO messages are sent way more often than ANS_KEY messages,
   thereby keeping the address information fresh. Previously, if the UDP
   situation were to change after the ANS_KEY message was sent, the
   sender would virtually never get the updated information.

 - Once a node puts address information in an ANS_KEY message, it is
   never changed again as the message travels through the metagraph; in
   contrast, UDP_INFO messages behave the opposite way, as they get
   rewritten every time they travel through a node with a confirmed UDP
   tunnel. The latter behavior seems more appropriate because UDP tunnel
   information becomes more relevant as it moves closer to the
   destination node. The ANS_KEY behavior is not satisfactory in some
   cases such as multi-layered graphs where the first hop is located
   before a NAT.

Ultimately, the rationale behind this whole process is to improve UDP
hole punching capabilities when port translation is in effect, and more
generally, to make tinc more reliable in (very) hostile network
conditions (such as multi-layered NAT).