fix the reply of rpc_alter_context

fix the reply of rpc_alter_context
OpenPrinterEx is now decoding correctly the query
most of the EnumXXX use the new_buffer struct.
check the (un)marshalling return code.

conclusion: still a long way to go. all the client code has to be
rewritten, and I still wonder how to implement correctly the notify stuff.
(This used to be commit 3d6d3863751787b08d40268c83221add1487a5c9)