Very encouraging patch. In this patch I've managed to surmount a really boring
obstacle. There are several more hurdles to go, but this makes the heart glad.

* We now have local endpoint-contexts for channel endpoints.
* We now also have local region metadata for region threads in userspace.
* Fixed negligence in the calculation of rdata size. We now properly take the
  udi_secondary_init_t::rdata_size for each region into account.
* Each region thread now allocates its own rdata, and this is no longer done
  in the DriverInstance's main thread.

* spawnInternalBindChannel and spawnChildBindChannel both now return pointers to
  the child-endpoint of the newly spawned channel. This enables us to construct
  our local userspace metadata.
* We no longer store rdata information within fplainn::Region. This is offloaded
  to userspace.
* New syscall: fplainn::Zudi::setEndpointPrivateData(), used to set a kernel
  endpoint's private data pointer. This pointer is passed in all sChannelMsg
  objects to driver regions, and is used by the region thread to quickly gain
  a context-sensitive pointer to the channel endpoint's data.