Noded: Consider certificate chain in callback

Noded: Consider certificate chain in callback

This patch significantly changes the callback that is
called upon receiving an incoming SSL connection. Since
this callback is called not only with the certificate
that the client sends, but also (in some implementations)
with the entire certificate chain of the client
certificate.

In our case, the certficate chain contains
the client certificate and the server certificate as
the one that signed the client certificate. This means
that we have to accept the server certificate, but only
if we receive it with the 'depth' greater than 0, meaning
that this is part of the chain and not the actual
certificate. If the depth value is 0, we can be sure
to have received the actual certficate and match it
against the list of master candidate certificates as
before.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>