Fix bug causing exception: "'Disembargo' of type 'senderLoopback' sent to an object that does not point back to the sender."

The problem happened when pipelined calls were made on a promised capability, but then that capability turned out to be null. The promise resolving code incorrectly interpreted this as the remote promise having resolved to a local capability (because the "null" stub capability looks local), and so it would send a Disembargo message to flush the pipeline as required. However, the remote end would receive this Disembargo message and find it is addressed to a null capability, not a capability pointing back to the sender. This was treated as a protocol error, causing the receiver to close the connection.

The solution is to explicitly identity "null" capabilities so that we can distinguish this case. This change also has the benefit that now when you copy a null capability between messages with foo.setCap(bar.getCap()), the pointer will be set null in the destination, rather than becoming a reference to a local broken capability.

Thanks to David Renshaw for narrowing this down.