Make (EC)DH key exchange actually provide perfect forward secrecy.

Without SSL_OP_SINGLE_(EC?)DH_USE, the SSL library will use the
same in-memory DH keypair across multiple sessions. This makes
sessions easier to break than they otherwise should be, because
an adversary can compromise (one of) the server's DH private keys,
combine it with each client's public DH key (from e.g. recorded
traffic) in the same way DH key exchange works, and derive the
session key in the same way, thus enabling them to decrypt the
entire session in the same way as if they had compromised the RSA
private key and DH had not been used. This defeats the point of
using (EC)DH. With these options, the library will generate a new
keypair for every session, discarding them before the next session.