BUFQ_CANCEL(queue, element) removes the specified element previously queued
on the queue. It returns NULL if it was not found on the queue and the
element if it was successfully removed.
Run trough tech-kern and changed name from BUFQ_REVOKE() by suggestion of
Jason Thorpe.
- rather than embedding bufq_state in driver softc,
have a pointer to the former.
- move bufq related functions from kern/subr_disk.c to kern/subr_bufq.c.
- rename method to strategy for consistency.
- move some definitions which don't need to be exposed to the rest of kernel
from sys/bufq.h to sys/bufq_impl.h.
(is it better to move it to kern/ or somewhere?)
- fix some obvious breakage in dev/qbus/ts.c. (not tested)
