SSL_CTX_set_ecdh_auto() for TLS-wrapped server configurations to
enable ECDHE. Otherwise, manually offer curve p256.
Client connections should automatically use ECDHE when available.
(thanks Daniel Kahn Gillmor)
git-svn-id: file:///svn/unbound/trunk@3452 be551aaa-1e26-0410-a405-d3ace91eadb9