devantech-eth008: first driver implementation, relay output only

devantech-eth008: first driver implementation, relay output only

Implement common support code to communicate raw binary requests and
responses over TCP sockets, and ETH008 specific requests to control
up to 8 relays as well as getting the card's supply voltage.

Other cards with differing channel counts, and with features beyond
relay control, have not been available during creation of this driver.
Their addition is prepared but untested. Password protection is not
implemented, but outlined in developer comments.