DIOC{G,S}STRATEGY not implemented for all disk devices

[Alan Barrett <apb%cequrux.com@localhost>]

Why don't the DIOC{G,S}STRATEGY ioctls work for all disk devices?
There seem to be several almost-identical implementations, in
sys/dev/dksubr.c, sys/dev/ld.c, and sys/dev/ata/wd.c; an implementation
in sys/dev/scsipi/cd.c that always returns EIO; and implementations in
sys/arch/xen/xbd.c and xbd_xenbus.c that return EOPNOTSUPP.

When I try to use "dkctl sd0 strategy" I get ENOTTY ("dkctl: /dev/rsd0d:
DIOCGSTRATEGY: Inappropriate ioctl for device"), so the implementation
in dksubr.c is apparently not being used as a fallback when the sd
driver doesn't provide its own implementation.

--apb (Alan Barrett)