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 <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/13973)