SSL_{CTX}_set_tmp_ecdh() allows to set 1 EC curve and then tries to use it.  On
the other hand SSL_{CTX_}set1_curves() allows you to set a list of curves, but
only when SSL_{CTX_}set_ecdh_auto() was called to turn it on.

Reviewed-by: Dr. Stephen Henson <steve@openssl.org>