Bugs:
  - replace memfd ioctls with shmem memfd_create() + fcntl() logic

Features:
  - test kdbus_match_bloom() multi-generation bloom masks, we need to
    be sure it works with old kernels when we update userspace tools

  - make sure that our domain/kuid/pidns creds passing/deny logic
    based on the PID namespace is what we need; interaction with userns?

  - 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
     - domain creation
     - ep prefixing
     - policy, policy for custom endpoints

  - attach seclabel to well-known names
  - attach view/talk/own access policy to names
  - support policy name wildcards like foo.bar.*

  - also attach queued names to message metadata?

  - support the creation of anonymous buses?

  - actually return compatible/incompatible flags to users

  - test message budget limits

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