Add OCSP_resp_get1_id() accessor

Adding a get1 style accessor as brought up in mailing list post
https://mta.openssl.org/pipermail/openssl-users/2016-November/004796.html



Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1876)