s3-rpc_server: Store the ndr syntax id in the pipe context.

s3-rpc_server: Store the ndr syntax id in the pipe context.

The client tell us in the rpc bind to which rpc service it wants to
connect. We did set the p->syntax earlier by guessing to which pipe name
it connects, but we don't know to which rpc service it wants to bind
until we read the first packet.