Add 'devctl delete' that calls device_delete_child().

'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