basic command/data phase implemented.
give FloppyDevice something to do
first command: Calibrate drive status implemented
add Check Interrupt Status command, fix coding issue that prevents
single-byte commands.
on invalid command, reset command state
more work, added IRQ signalling.
FDC DOR write function should be a member of the FloppyController class.
add code to reject Seek Relative command (as most FDCs do)