Add queueing to np_read_state, simulate message-type named pipes.

Add queueing to np_read_state, simulate message-type named pipes.

The problem with msg-type pipes is that we have to return short reads when a
message ends before the read request. When reading from the unix domain socket,
the message limits are lost. So we would happily return more than a message,
which confuses for example the s4 rpc client horribly. I'd expect other np rpc
clients also to blow up over this.

The real solution is to properly implement a two-byte length field per message
on the unix domain socket, but this requires more changes there. And as we
right now only serve DCE/RPC over the named pipes, this implements a hack that
looks into the fragment headers to figure out hdr.frag_len.