Convert the USB ethernet drivers to use mutexes. Also convert
usb_ethersubr.c. This module maintains two queues for packets which
are each protected with one mutex. These are all the changes I can
do for now. Removing the USBD_NO_TSLEEP flag doesn't work yet: when
I tried it, the system would usually freeze up after a NIC had been
operating for a while. The usb_ethersubr module itself ought to
go away; this is the next thing I need to test.