Signature algos: allow having identical digest in params

Signature algos: allow having identical digest in params

The flag_allow_md prevents setting a digest in params however
this is unnecessarily strict. If the digest is the same as the
one already set, we do not return an error.

Fixes #16071

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16076)