autoconf(9): New localcount-based device instance references.

device_lookup_acquire looks up an autoconf device instance, if found,
and acquires a reference the caller must release with device_release.
If attach or detach is still in progress, device_lookup_acquire waits
until it completes.  While references are held, the device's softc
will not be freed or reused until the last reference is released.

The reference is meant to be held while opening a device in the short
term, and then to be passed off to a longer-term reference that can
be broken explicitly by detach -- usually a device special vnode,
which is broken by vdevgone in the driver's *_detach function.

Sleeping while holding a reference is allowed, e.g. waiting to open a
tty.  A driver must arrange that its *_detach function will interrupt
any threads sleeping while holding references and cause them to back
out so that detach can complete promptly.

Subsequent changes to subr_devsw.c will make bdev_open and cdev_open
automatically take a reference to an autoconf instance for drivers
that opt into this, so there will be no logic changes needed in most
drivers other than to connect the autoconf cfdriver to the
bdevsw/cdevsw I/O operation tables.  The effect will be that *_detach
may run while d_open is in progress, but no new d_open can begin
until *_detach has backed out from or committed to detaching.

XXX kernel ABI change to struct device requires bump -- later change
will make struct device opaque to ABI, but we're not there yet

(riastradh)