Legacy-free PC hardware do not have a real PS/2 keyboard controller, but

rather have the USB HCI emulate it during boot, while legacy mode is enabled.

This causes pckbd0 to attach as the console device, but is lost as soon as
the USB HCI driver attaches.

The disappearance of the emulated PS/2 controller can however be detected
in pckbc(4) - which is supposed to attach after [eou]hci(4), with the controller
refusing to ack commands and replying ``please resend'' instead.

In that case, the kernel will now no longer attach pckbd, and will perform a
new console input device selection, allowing the (real) usb keyboard to
become the console.

Thanks to krw@ for countless tests on legacy-free hardware; also tested on
more conventional hardware by naddy@ and I.

Only amd64 and i386 platforms are affected by this change.

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