USB backend: look for ipp-usb

[michaelrsweet]

Now that ipp-usb is advertising a DNS-SD service when it is connected to a device, we need to add a lookup for USB printers with a protocol class 4 (IPP-USB) endpoint.

The current code creates a service name of "BBPP._ipp-usb._tcp" where BB is the bus number in hex and PP is the port number in hex. If we find it, then ipp-usb is active for the printer and we shouldn't list it as a legacy USB printer.