Fix a compiler warning when checking for DH mode

As noted by others, and the compiler, rep->choice can't ever be set to
choice_pa_pk_as_rep_draft9_dhSignedData, so check for it in the correct
structure.
Fix draft9 Diffie-Hellman client requests

If we're creating a draft 9 request, and the user set flag_RSA_PROTOCOL
to "no", use Diffie-Hellman key agreement.

We switch the default from explicitly defaulting to using DH to a having
a per-protocol-version default to keep the traditional behavior if the
user doesn't specifically request one or the other.
Return DH data correctly in draft 9

When processing a draft 9 request, return the DH key agreement data in
the format specified in draft 9, which differs from the RFC4556 version.
Parse DH PA-PK-AS-REP-OLD padata correctly

Try to handle the differences between the draft 9 version of
Diffie-Hellman key agreement and the RFC4556 version.  Incomplete.