SSL: speedup loading of configs with many ssl servers.

The patch saves one EC_KEY_generate_key() call per server{} block by
informing OpenSSL about SSL_OP_SINGLE_ECDH_USE we are going to use before
the SSL_CTX_set_tmp_ecdh() call.

For a configuration file with 10k simple server{} blocks with SSL enabled
this change reduces startup time from 18s to 5s on a slow test box here.