Use extra buttons of Kensington Slimblade Trackball

[Lee, Chung-Yeol]

 Hi, 

 While I'm searching how to use the extra buttons in Kensington Slimblade
Trackball, I made a patch for myself based on the linux driver.

I confirmed it works as intended. However, I have no experience with the FreeBSD
kernel and I'm not sure that I modified it in a correct way. -- especially,
I don't know should I use usb_quirks?

 Can somebody check the attached diff file so that I can send problem report?

 Thanks.