s3:rpc_server: Add termination function and data to dcerpc_ncacn_conn

s3:rpc_server: Add termination function and data to dcerpc_ncacn_conn

In preparation to remove the named_pipe_client structure, this function
will be called from ncacn_conn destructor passing the private data pointer
as argument. It is mainly used by preforked childs in "process model" to
attend the next client as soon as the previous client ends the connection.

Signed-off-by: Samuel Cabrero <scabrero@suse.de>
Reviewed-by: Andreas Schneider <asn@samba.org>