Extract compression form in EC_KEY_oct2key().

Extract compression form in EC_KEY_oct2key() instead of manually in the
ASN.1 code. For custom curves do not assume the initial octet is the
compression form: it isn't for X25519 et al.

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