Fix bugs in X509_NAME_ENTRY_set

The wrong "set" field was incremented in the wrong place and would
create a new RDN, not a multi-valued RDN.
RDN inserts would happen after not before.
Prepending an entry to an RDN incorrectly created a new RDN

Anything which built up an X509_NAME could get a messed-up structure,
which would then be "wrong" for anyone using that name.

Thanks to Ingo Schwarze for extensive debugging and the initial
fix (documented in GitHub issue #5870).

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/5882)