Avoid only exact duplicates when creating the accepted CA names list

This avoids situations where third party client is unable to recognize
that the client certificate was issued by the same CA with name differring
only by case or insignificant characters.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4731)