Run SMB code as SCHED_FIFO.

Especially since the DAC SPI traffic is done by GPIO bit-banging, make sure to run the controller in the SCHED_FIFO class.

This should be safe: the code is a select loop waiting for TCP command packets and breaking out for a 1Hz update loop. Components of the steps of that loop are what need to be made non-interruptable; there is plenty of idle time after the step for us to do it all in one single step.

Will abuse sudo, since I am not energetic enough to figure out robust use of linux capabilities, etc.