Only support >= 256-bit elliptic curves with ecdh_auto (server) or by default (client).

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: Kurt Roeckx <kurt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>

CHANGES

@@ -4,6 +4,11 @@
 
  Changes between 1.0.2a and 1.0.2b [xx XXX xxxx]
 
+  *) Only support 256-bit or stronger elliptic curves with the
+     'ecdh_auto' setting (server) or by default (client). Of supported
+     curves, prefer P-256 (both).
+     [Emilia Kasper]
+
   *) Reject DH handshakes with parameters shorter than 768 bits.
      [Kurt Roeckx and Emilia Kasper]