Don't accept connections for servers already beeing linked

If two servers try to link each other, there was a time frame that
could result in one connection overwriting the other, e. g. the incoming
connection overwriting the status of the outgoing one. And this could
lead to all kind of weirdness (even crashes!) later on.

So now such incoming connections are dropped. But this most probably
prevents the two servers from linking until timing changes somehow
(network latency?) because each server drops the incoming connection of
the other one, so no connection survives in the end.

But this has to be addressed by an other patch ...

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