"poll" flag when called from ld_shutdown().
This is the infrastructure part of kern/38655 - in itself it doesn't
fix the panic referenced in that PR.
XXX: At least ld_twa.c and perhaps ld_iop.c and ld_icp.c need to
check for this new poll flag and do something useful.
- 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)
cegger