[USB] usbcore avoids RNDIS configuration.

Modifies the "choose a configuration" heuristic to recognize the other
case that 2.4 handled:  RNDIS, a MSFT protocol that's sometimes used as
the first/default configuration (as specified by MSFT) on cable modems.
CDC Ethernet is vendor-neutral, and preferred (except by MSFT).

The initial version didn't catch RNDIS because it cloaks its ethernet
links as a kind of CDC ACM (modem) device.