Turn on keyboard interrupts by default

[ausbin]

I don't think this causes any problems, but we should test it thoroughly and make sure.

The default interrupt handler is just an RTI which seems harmless: https://github.com/gt-cs2110/lc3tools/blob/master/src/backend/lc3os.cpp#L715

Why am I asking for this? We wanted to show an interrupt example, but we had to muck around with bit 14 of the KBSR which is making students anxious ahead of the final