freebsd_network: Add missing NULL check in get_usb_device_attach_arg.

May fix #18418.

Add missing NULL checks to USB drivers.

Should fix #17276, #17846.

freebsd_network: Do not deinitialize USB if we never initialized it.

Fixes some "mutex not owned" KDLs seen on deinitialization.

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: Use a dedicated USB mutex instead of &Giant.

Fixes #17804.

freebsd_network: Check for errors in usbd_transfer_submit.

This way, transfers that fail to queue will have their callbacks
actually invoked.

Fixes hangs when trying to disable unplugged USB WiFi devices.

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.