Handle PKINIT DH replies with no certs

Handle PKINIT DH replies with no certs

If a PKINIT Diffie-Hellman reply contains no certificates in the
SignedData object, that may be because the signer certificate was a
trust anchor as transmitted to the KDC.  Heimdal's KDC, for instance,
filters client trust anchors out of the returned set of certificates.
Match against idctx->trustedCAs and idctx->intermediateCAs to handle
this case.  This fix only works with OpenSSL 1.0 or later; when built
against OpenSSL 0.9.x, the client will still require a cert in the
reply.

Code changes suggested by nalin@redhat.com.

ticket: 7183