Add no signing flag.

Add a flag to EC_METHOD for curves which do not support signing.
New function EC_KEY_can_sign() returns 1 is key can be used for signing.
Return an explicit error is an attempt is made to sign with
no signing curves.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Emilia Käsper <emilia@openssl.org>