This is a follow-up to 5fa29797910346fc0c54829bd979856e83b9b7ea .

PR:		256311
Reviewed by:	imp
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D29867

(cherry picked from commit 315674fb6acc4fa54cf82de3863c349c5a71f498)

MFC after:	1 week

Reviewed by:	imp (previous version), jhb (previous version)
Sponsored by:	Mellanox Technologies
MFC after:	2 weeks
Differential revision:	https://reviews.freebsd.org/D19646

Reviewed by:	rgrimes
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D19485

PR:		219395
Submitted by:	Tiwei Bie

Add a new 'clear driver' command for devctl along with the accompanying
ioctl and devctl_clear_driver() library routine to reset a device to
use a wildcard devclass instead of a fixed devclass.  This can be used
to undo a previous 'set driver' command.  After the device's name has
been reset to permit wildcard names, it is reprobed so that it can
attach to newly-available (to it) device drivers.

MFC after:	1 month
Sponsored by:	Chelsio Communications

'devctl delete' can be used to delete a device that is no longer present.
As an anti-foot-shooting measure, 'delete' will not delete a device
unless it's parent bus says it is no longer present.  This can be
overridden by passing the force ('-f') flag.

Note that this command should be used with care.  If a device is deleted
that is actually present it can't be resurrected unless the parent bus
device's driver supports rescans.

Differential Revision:	https://reviews.freebsd.org/D6019

The BUS_RESCAN() method rescans a single bus device checking for devices
that have been added or removed from the bus.  A new 'rescan' command is
added to devctl(8) to trigger a rescan.

Differential Revision:	https://reviews.freebsd.org/D6016

- Add missing devctl_set_driver entry to namelist in devlist(3).
- Fix sorting of function prototypes in devlist(3).

MFC after:	3 days