Expand whitelist for use of "offline-report"

Expand whitelist for use of "offline-report"

The old behavior unconditionally cleared the "offline-report"
printer-reason unless the backend was 'usb:' or 'ippusb:' with the
assumption that these were the only two direct-connect USB backends
in use, with everything else being remote.

This is definitely not the case; putting aside proprietary 3rd-party
backends, there is Gutenprint's USB backend, which identifies itself
with 'gutenprint52+usb:' or 'gutenprint53+usb:' depending on the version.

So, generalize the rule to search for the string 'usb:' in the uri
instead, which will allow all three/four usb backends to use the
'offline-report' printer-reason.

(FWIW, I'm not happy with this approach; It seems to me that the
 CorrectSolution(tm) is to track the device-class returned by the backend
 at discovery time, and ignore 'offline-report' for anything other than
 'direct', but that's going to be a lot more involved..)