new-bus: Fix some shortcomings in disabling devices via hints

A device can be disabled via a hint after it is probed (but before it
is attached). The initial version of this marked the device disabled,
but left the device "alive" meaning that dev->driver and dev->desc
were untouched and still pointed into the driver that probed the
device. If that driver lives in a kernel module that is later
unloaded, device_detach() called from devclass_delete_driver() doesn't
do anything (the device's state is DS_ALIVE). In particular, it
doesn't call device_set_driver(dev, NULL) to disassociate the device
from the driver that is being unloaded.

There are several places where these stale pointers can be tripped
over. After kldunload, invoking the sysctl to fetch device info can
dereference dev->desc and dev->driver causing panics. Even without
kldunload, a system suspend request will call the device_suspend and
device_resume DEVMETHODs of the driver in question even though the
device is not attached which can cause some excitement.

To clean this up, more fully detach a device that is disabled by a
hint by clearing the driver and setting the state to DS_NOTPRESENT.
However, to keep the device name+unit combination reserved, leave the
device attached to its devclass.

This requires a change to 'devctl enable' handling to deal with this
updated state. It now checks for a non-NULL devclass to determine if
a disabled device is in this state and if so it clears the hint.
However, it also now clears the devclass before attaching the device.
This gives all drivers an opportunity to attach to the now-enabled
device.

Reported by: adrian
Discussed with: imp
Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47691

(cherry picked from commit b4c700fa7cca7d3e4da84dd417fc3efc42788a52)