r4618: - tidied up the alter_context client code a bit

r4618: - tidied up the alter_context client code a bit

- there is no alter_nak or alter_ack packet, its all done in an
  alter_response

- auto-allocated the contex_ids

- tried to fix up the dcom code to work again with
  alter_context. Jelmer, please take a look :)
(This used to be commit dd1c54add8884376601f2f8a56c01bfb8add030c)