Set ECDHE curve independently from DHE parameters

Hi,

ECDHE doesn’t require DHE parameters so the logic in core/ssl.c isn’t correct.

You can test it easily: run a uwsgi server with a certificate without DH params using current master and with this patch applied (use `'AESGCM'` for ciphers).  Connect using `openssl s_client -connect host:port` to each.   In the current behavior you’ll get `AES256-GCM-SHA384`, with the patch `ECDHE-RSA-AES256-GCM-SHA384`.

I wasn’t able to locate any tests for this code, so I haven’t added any but I took the liberty to check the return value. :)

BTW, if the DHE-behavior is documented somewhere, I was too dumb to find it.