ukbd locking update

[Andriy Gapon]

I completing a patch that changes some locking in ukbd to account for
SCHEDULER_STOPPED and for other realities of the code.

As a preview I would like to share couple of observations that had their effect
on the patch.

1. Acquiring Giant in device_attach, _detach in similar newbus method
implementations should be redundant because those are already executed with
Giant held.  That's done either by the general newbus code or via
usbd_enum_lock() when the operations are executed in the USB explore thread.

2. As discussed before:
if (!mutex_owned(&Giant))
	mutex_lock(&Giant)
this pattern does not make sense, because the Giant is recursive and can be
simply acquired without any check.

Do you agree?
-- 
Andriy Gapon