Begin implementation work on RPC connect points
This is the first stage of #1528 (closed). It provides a backend implementation for RPC connect points in sufficient detail to:
- Handle filesystem operations
- Parse connect points
- Initialize cookie authentication as needed
- Open or bind sockets as required.
There will probably be some conflict with !2615 (merged); I'll fix that once it lands.
It does not handle all of #1528 (closed); see that ticket for next steps.