Basic transmit path works. Userspace example updated to show how to use
it. The e1000 driver doesn't have RX/TX capability yet. Cleaned up all
of the RX/TX code across the data path and split some of the NIC driver
operations (port_ops) (read/write becomes recv/clean/xmit). 

There is still no blocking on xmit, so contention on transmit will cause
the arbiter to spin pretty hard (to be fixed later, but not an issue
right now).