Test against both OpenSSL and GnuTLS.
Don't use a proxy. It's not particularly useful here, and would complicate
figuring out port numbers.
Clean up compile-time requirements in dtls_client.c: any certificate-based
key exchange is ok, so don't insist on built-in RSA.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>