FCPPluginReply: Allow construction of replies which contain a "Success"
field
- This serves as the inverse of the "Success" field which we parse in freenet.node.fcp.FCPPluginMessage.
- It will be used by FCPPluginClient.send() to produce server-to-client messages.
- The FIXMEs at the "success" variable of FCPPluginMessage apply, so this commit adds mentioning of FCPPluginReply there.