Add support for SSL_CTX_set1_curves

[Sytten]

Couple of things couple be added to make life easier to work with curves.
In the boring ssl crate, they have a set_curves and set_curves_list (https://docs.rs/boring/latest/boring/ssl/struct.SslContextBuilder.html#method.set_curves) with a safe SslCurve object for the NID.
Pretty sure it would be simple to add here, let me know if you want a PR for that.