EC: Reverse the default asn1_flag in a new EC_GROUP
EC: Reverse the default asn1_flag in a new EC_GROUP

The default was OPENSSL_EC_NAMED_CURVE, but that's not true until a
curve name has been set, so we change the initial value to
OPENSSL_EC_EXPLICIT_CURVE and let EC_GROUP_set_curve_name() change it
to OPENSSL_EC_NAMED_CURVE.

Submitted by Matt Caswell

Reviewed-by: Dmitry Belyavskiy <38c64d6e24766247aad56af3a6ddb0056ad36a44@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/13973)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl