Cleanups to RPC code.

This is extracted from a significantly longer RPC change that I put down a couple of weeks ago.

It deletes some unused code, uses Issue and Book a little more, and fixes some long lines and indentation.

Feel free to use this or ignore it!
The primary use for this is to allow people to add their own RPC handlers without having to maintain changes to existing files. The linked list is only ever modified by objects that construct before the RPC server is actually operating. This is definitely a bit of a hack, and I'd be quite happy to replace it with something better.