RPC: Add a method on RpcSession to find the location of the socksport

In order to provide a sensible implementation for "make a connection", we need an RPC method that tells us where the socksport is.

One complication here is that the RpcSession object is defined in arti-rpcserver, whereas the socksport is only available in arti. We'll need to do some refactoring here so that we can associate the information in a useful way, and keep it up-to-date if arti is reconfigured.