Cleanup, mostly removing remaining references to mouse
Making 4 joys actually work by disabling serial

We're limited to 6 endpoints, so I added a #define option
(ENABLE_SERIAL) that switches between 4 joys and no serial, and
2 joys and a serial port.

Also added a num_joys() function to the MultiJoystick API, so that
programs can know how many joysticks they're working with and
adjust accordingly.

Future possiblities:
 - Multiple USB configurations, for runtime switching?
 - Also allow 3 joysticks, serial, and no keyboard
Make axis() 1-based, for consistency with button()