r4616: the first phase in the addition of proper support for

r4616: the first phase in the addition of proper support for
dcerpc_alter_context and multiple context_ids in the dcerpc client
library.

This stage does the following:

 - split "struct dcerpc_pipe" into two parts, the main part being "struct dcerpc_connection", which
   contains all the parts not dependent on the context, and "struct dcerpc_pipe" which has
   the context dependent part. This is similar to the layering in libcli_*() for SMB

 - disable the current dcerpc_alter code. I've used a #warning until i
   get the 2nd phase finished. I don't know how portable #warning is, but
   it won't be long before I add full alter context support anyway, so it won't last long

 - cleanup the allocation of dcerpc_pipe structures. The previous code
   was quite awkward.