MFC 305034: Implement 'devctl clear driver' to undo a previous 'set driver'.

MFC 305034: Implement 'devctl clear driver' to undo a previous 'set driver'.

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.

Sponsored by:	Chelsio Communications

git-svn-id: svn://svn.freebsd.org/base/stable/10@306533 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f