Ensure that TrySetECDHNamedCurve is always called, enabling ECDHE ciphersuites

Ensure that TrySetECDHNamedCurve is always called, enabling ECDHE ciphersuites

Moves the call to TrySetECDHNamedCurve above the early abort when the server default protocols are selected.

This ensures that when SslStream is used as a TLS server that the ECDHE ciphersuites are available for "Perfect Forward Secrecy".

By using an SslStream-based TLS server and the openssl s_client utility as a client, verified that the ciphersuite went from ECDH-ECDSA-AES256-GCM-SHA384 to ECDHE-ECDSA-AES256-GCM-SHA384 (and AES256-GCM-SHA384 to ECDHE-RSA-AES256-GCM-SHA384 for RSA).

Commit migrated from https://github.com/dotnet/corefx/commit/18720f9b286770a08f59e866e4da220d3aadaf29