Relay-control service

The relay control has been split off to a service running as root, so the main flight software can run in user-space again.  Relay commanding has been tested on both flight computers.

Unfortunately, the communication is currently implemented as single-direction, so there is no way to query the service for the current state of the relay bank (or rather what it thinks the state is, since we have no ability to actually query the board).

Merging immediately