freebsd_network: Implement USB routines needed by Ralink USB modules.

Tested by nephele with a RT2573. The device starts and scans for networks,
but hits some USB errors probably related to buffer sizing which I'm
not quite sure what the cause of could be. More work (by someone who
owns these devices) may be required.

freebsd_network: Introduce USB-handling code.

It is not yet wired in to the rest of the compatibilit layer;
that will happen in the next commits.

The bulk of the implementation is in usb.cpp; most of the
rest of the new files are copied from FreeBSD to varying degrees.