Add ucc(4), a driver for USB HID Consumer Control keyboards. Such

Add ucc(4), a driver for USB HID Consumer Control keyboards. Such
keyboard is a pseudo device which is used to expose audio and
application launch keys. My prime motivation is to get the volume mute,
increment and decrement keys to just work on my keyboard without the
need to use usbhidaction(1).

Looks reasonable to kettenis@ mpi@ and ok jcs@