OpenSSL: Loop over DTLS ciphersuites looking for the one we asked for.

OpenSSL: Loop over DTLS ciphersuites looking for the one we asked for.

As of OpenSSL 1.1.1, the trick of using SSL_CTX_set_cipher_list() and then
expecting only the one ciphersuite to be present in what we get back from
SSL_get_ciphers(), is no longer working. It now always returns the TLSv1.3
ciphers, even though we don't have DTLSv1.3 yet.

Reported as https://github.com/openssl/openssl/issues/8004 but probably
not going to change; the most likely outcome there is that I'm told that
I'm Doing It Wrong™ and a different approach is suggested.

In the meantime, just loop over the results and pick the one that we
actually asked for.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>