Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add uep(4), driver for USB onscreen touch panel from eGalax.

The driver is stub. It just creates device entry and feeds
reassembled packets from hardware into it.

If in future we would port wsmouse(4) from NetBSD, or make
sysmouse(4) to support absolute motion events, then the driver
can be extended to act as system mouse. Meanwhile, it just
presents a /dev/uep0, that can be utilized by X driver, that
I am going to commit to ports tree soon.

The name for the driver is chosen to be the same as in NetBSD,
however, due to different USB stacks this driver isn't a port.