enable generic interface (/dev/ngX) for unknown command sets v3

[Christoph Hellwig]

Hi all,

this is a version of the patch from Joel rebased on a bunch of cleanups
from me that I think make the layering a lot cleaner.  Let me know what
everyone things.

Changes since v2:
 - add a patch to fix a previous regression in handling of unsupported
   ZNS configs

Changes since v1:
 - log consistent messages when only passthrough is supported for
   namespaces