Add uatp(4), a driver for USB Apple trackpads.

This is a work-in-progress driver for USB trackpads found in Apple
laptops since 2005, theoretically covering more models than pbms(4)
and supporting more features.  However, the motion smoothing and
acceleration formulae are still pretty sketchy, and I have tested
this only on one model of MacBook from 2006.  Feedback welcome.

Should the smooting, acceleration, tapping, &c., be done in userland?
Probably, but we don't have the necessary interface for that to work
well yet -- wsmouse isn't enough as is.

(riastradh)