fix assertion: usb_transfer_chain_active actually returns OK when still active
IPC_DEVICEMEM flag = no cache, no buffer
propagate USE_VMM to userlib Makefile
add "attached" and "flags" fields to usb_device_t
ehci_td_traversal_next requires physical address as well in order to setup next_td
fix a few thinkos -- was clearing startTD repeatedly instead of clearing fresh TDs
future smsclan USB ethernet module (but mostly testing USB stuff right now)