gensec: Assert that we have not been subject to a downgrade attack in DCE/RPC clients

gensec: Assert that we have not been subject to a downgrade attack in DCE/RPC clients

Because of the calling convention, this is the best place to assert
that we have not been subject to a downgrade attack on the negotiated
features.  (In DCE/RPC, this isn't a negotiation, the client simply
specifies the level of protection that is required).

Andrew Bartlett

(some formatting fixes)
Signed-off-by: Stefan Metzmacher <metze@samba.org>