Add supported elliptical curves (HTTPS/TLS)

[rugk]

This is related to #1144, but this issue is about the supported elliptical curves in ECDHE connection or ECDSA ciphers. In both cases elliptical curves are used.

Here is a current list of supported curves:
https://security.stackexchange.com/questions/31772/what-elliptic-curves-are-supported-by-browsers

Also discussed on the Let's Encrypt forum.

[vvianyy]

+1

[rugk]

Note that Chrome 50 e.g. supports x25519.

[Bisaloo]

+1

[JaneX8]

+1

[biergaizi]

See also: #3157

[socketpair]

+1