SSL: let a server choose the most appropriate curve for a client

OpenSSL 1.0.2 introduces SSL_CTX_set_ecdh_auto, which allows us
to stop using one specific temporary curve, and instead makes
the server negotiate the best curve.

Task-number: QTBUG-42925
Change-Id: I3a68f29030bdf04f368bfdf79c888401ce82bdd8
Reviewed-by: Richard J. Moore <rich@kde.org>