Tidy up certificate type handling.

The certificate types used to be held in a fixed length array or (if
it was too long) a malloced buffer. This was done to retain binary
compatibility. The code can be simplified now SSL is opaque by always
using a malloced buffer.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2733)