local address announcements

[Daniel Schall]

> > Assuming we have three nodes (A, B and C).
> > A acts as a server and has the keys of B and C.
> > B and C do not have each other's keys, only the key from A.
> > Nevertheless should B and C be able to establish a direct UDP
connection:
> > They first exchange keys with A to establish a meta connection.
> > A tells B, how C can be reached, and vice-versa.
> > After exchanging keys over the meta connection they both have with A, 
> > B and C are able to directly communicate.
> > This applies to the "standard" packet exchange as well as my 
> > multicast/local approach.
>
>   a
>  / \
> b  c
>
> For this to work, does also a need to have your version of tinc? Or is it
enough for b and c to be this special version?

A must have my version as well, because I changed the packet data structure.
I added another field, called flags, which is not encrypted, but gets sent
as well.