make raw USB printer devices accessible for lp

make raw USB printer devices accessible for lp

Starting from version 1.4, cups now uses libusb and printer USB devices instead
of the usblp generated /dev/usb/lpX ones. In order to not require the cups USB
backend to run as root now, change raw USB printer devices to be root:lp 0660,
similar to usblpX devices.

This might also enable the hplip backend to not run as root, since this has
always used raw device nodes.

https://launchpad.net/bugs/420015