Define power-up configuration for the PCM and persist it.

There are a handful of configuration parameters for the PCM which we need to post set and persist. The two main ones are the per-port powerup state (whether a port is energized when the PCM is reset/powered up), and the voltage thresholds for shutting ports down.

As with the focus motors and the turbo pump, we should set these every time we connect to the PCM. But we also need to persist the configuration in some nice and editable way.