BootKeyboard doesn't send ErrorKeyRollover when too many keys are pressed

[tlyu]

According to the HID spec, we should send ErrorKeyRollover in all positions when there are more (non-modifier) keys pressed on the Boot Keyboard than there are positions in the Boot Report. We currently do not.

[gedankenexperimenter]

Before making this change, we should really find out what all the common operating systems do in response to those reports.

[tlyu]

Before making this change, we should really find out what all the common operating systems do in response to those reports.

It's standard behavior required by the HID specification for boot protocol, and every other keyboard I've tested so far correctly sends ErrorKeyRollover if too many keys are pressed at once (which can be fewer than 6, on cheaper keyboards without a full diode matrix).