Convert legacy TLS client to tls_key_share.

Convert legacy TLS client to tls_key_share.

This requires adding DHE support to tls_key_share. In doing so,
tls_key_share_peer_public() has to lose the group argument and gains
an invalid_key argument. The one place that actually needs the group
check is tlsext_keyshare_client_parse(), so add code to do this.

ok inoguchi@ tb@