Change DTLS client cert request code to match TLS.

Change DTLS client cert request code to match TLS.

DTLS currently doesn't check whether a client cert is expected.  This
change makes the logic in dtls1_accept() match that from ssl3_accept().
From OpenSSL commit c8d710dc5f83d69d802f941a4cc5895eb5fe3d65

input + ok jsing@ miod@