TODO

Features:
  - make sure that our namespace/kuid/pidns creds passing/deny logic
    is what we want in the long run

  - use/prepare for something like this to manage the pool backing shmemfd:
      https://git.kernel.org/cgit/linux/kernel/git/minchan/linux.git/commit/?h=vrange-v10-rc5&id=0b40a69d4c5d9eb13352fd357d73a5fab3ee699d

  - figure out/ prepare for priority inheritance like mutexes for the sync call

  - check guards for all privileged-only operations:
     - activation
     - monitor
     - impersonation
     - name register
     - $UID prefix
     - ns creation
     - ep prefixing
     - policy, policy for custom endpoints

  - attach seclabel to names?
  - attach policy to names? Where/how to store names from policy but
    otherwise inactive names (name laceholders).
  - find a way to specify "name prefix wildcards" like:
     "com.expample.foo.* can be owned by uid foo"

  - also attach queued names to message metadata?

  - account and limit number of messages a connection can have in-flight
    for another connection, like a connection can have a maximum of 100
    messages in-flight, but only 10 of them for the same connection

  - allow to update the metadata subscription bit mask

  - support the creation of anonymous buses

  - actually return compatible/incompatible flags to users

External API:
  - rules for:
      -unknown items to ignore in userspace lib?
      -unknown items to ignore if SEND sees them?

  - review all different structures of custom ioctls and items if they
    can be further unified