[core] Allow other (D)TLS Curves/Groups beside P-256

[core] Allow other (D)TLS Curves/Groups beside P-256

Five years ago, commit 8e1b2ea enabled ECDHE for the DTLS server by hard-coding P-256. Released before that, OpenSSL 1.0.2, allows auto selection of the curve (P-256, P-384, and P-512). OpenSSL 1.1.x has this enabled on default, which adds groups/curves like X25519 and X448 automatically. This change here allows DTLS clients with a demand for Security Level 4 (192 bit) and 5 (256 bit) connecting to the DTLS server.