Miscellaneous Linux header file crud.
Fix DRM_WAIT_UNTIL to initialize (RET) on every exit.
Omit more needless #ifdef __NetBSD__.
Round of hacking on i915 framebuffers.
Rework Linux `work' to use NetBSD workqueues, not callouts.

Callers expect to be able to allocate in the workers, which callouts
don't allow.

Delayed work uses callouts only to delay enqueueing work.

Linux `workqueues' are still stubs.
First draft of genfb attachment for i915.