Don't mix up pre- and post-handshake verification of DTLS clients.

Commit db965c9b addressed TLS clients only.

When verifying DTLS clients, don't consider config blocks with CA
settings ('tls') which differ from the one used for verifying the
certificate chain.

Original issue reported and analysed by Ralf Paffrath. DTLS being
vulnerable reported by Raphael Geisser.

Addresses issue RADSECPROXY-43, CVE-2012-4523.