always generate key if (EC)DH is used

I've found that this option is recommended if (EC)DH is used.

Docs:
```
Always create a new key when using temporary/ephemeral DH parameters (see SSL_CTX_set_tmp_dh_callback(3)). This option must be used to prevent small subgroup attacks, when the DH parameters were not generated using ``strong'' primes (e.g. when using DSA-parameters, see dhparam(1)). If ``strong'' primes were used, it is not strictly necessary to generate a new DH key during each handshake but it is also recommended. SSL_OP_SINGLE_DH_USE should therefore be enabled whenever temporary/ephemeral DH parameters are used.
```