Describes how to initialise a subsession.
On the client side, the function RSubSessionBase::CreateSubSession() is
used to create a new subsession and register it with the server.
The client must already have established a session with the server. A reference
to the client side session handle must be passed to the CreateSubsession() function.
The operation code must be one which the server interprets as a request to create a sub-session.
Only the first three 32 bit elements of the message argument array can be used by the client. The fourth element is used to communicate the handle number of the new server side subsession object.. In subsequent client requests, the fourth element is always used to pass the handle number to the server.