MFC r303765 and r304571:
Keep a reference count on USB keyboard polling to allow recursive
cngrab() during a panic for example, similar to what the AT-keyboard
driver is doing.

Make the UKBD USB transfers double buffered and set them up one by one,
so they are memory independent which allows for handling panics
triggered by the keyboard driver itself, typically via CTRL+ALT+ESC
sequences. Or if the USB keyboard driver was processing a key at the
moment of panic. Allow UKBD to be attached while keyboard polling is active.