local address announcements

[Daniel Schall]

> > > My algorithm is able to sign multicast packets only if a key 
> > > exchange has occurred before.
> > > (key exchange happens when you first contact the other node.) 
> > > Therefore, if you ping the nodes once (over the tinc interface), the 
> > > > keys should get exchanged and multicast signing should work.
> > > Yes, this is an issue that could easily be improved.
> > > Anyway, the code is still beta, it works most of the time, but I 
> > > experience some strange behavior too.
> > > Sometimes, nodes do not see each other, but I did not yet find out,
why.
> >
> > Tried it a couple of times and it seems it doesn't work.
> > I verified that both systems see each others broadcasts.
> > Furthermore I pinged both from within each other on their tinc address.
> > Didn't help.
>
> In the meantime, I added a key-request, in case the key is not yet
exchanged.
>
> Besides that, I am experiencing the same behavior:
> My two nodes exchange their local addresses, but packets sent over the
tinc interface get lost between the two.
> I did not find out, why this is happening, but I am on it.

There was a problem with verifying remote HMACs.
I forgot to adjust the packet length (to include the newly added flags).
Therefore, verifying HMACs in function try_harder always failed and packets
could not be allocated to nodes.

I fixed that issue, and tinc should be more stable now, although adjacent
nodes do not see each other 100% of the time.
Sending/receiving UDP packets sent to the public interfaces works though.

To reflect the change in the protocol, I increased the protocol version to
18, making it incompatible with previous tinc versions.