IRpcChannelBuffer_GetBuffer may put extra data onto the wire for ORPC
calls.

idenitfy what the code contained does - it is a number of helpers for
generated client/server code.

functions to cproxy.c since both sets of functions depend on the
implementations of the standard stubs and proxies.

which seems to be more appropriate and matches the errno translation
in ntdll.

This shows that we have a vtbl ptr at (void**)This-2 that is passed to
the base interface's IRpcStubBuffer_Connect.  This vtbl should
presumably forward calls to the real object's vtbl.  We can't simply
pass the real object to the base interface's stub, as the real
object's QI may not provide us with the base interface.