Also reorder preferences to prefer prime curves to binary curves, and P-256 to everything else.

The result:

$ openssl s_server -named_curves "auto"

This command will negotiate an ECDHE ciphersuite with P-256:

$ openssl s_client

This command will negotiate P-384:

$ openssl s_client -curves "P-384"

This command will not negotiate ECDHE because P-224 is disabled with "auto":

$ openssl s_client -curves "P-224"
Reviewed-by: NKurt Roeckx <kurt@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>