freebsd_network: Use the real usb_endian.h.

Turns out some FreeBSD drivers use these macros on types other
than the ones declared in this header and expect them to behave
as array accessors, so we really do have to keep the originals.

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.