lx810l, ap2000: implement working devices

Improve Epson LX-810L and ActionPrinter 2000 up to a point where they
actually work. Devices boot and enter main input loop, but input is
not yet implemented. It is possible to test the printers by toggling
the Line Feed button (press 'L'), resetting the machine, toggling the
Line Feed button again, and holding the Online button down (press 'O').

The code has also been split from lx800.