Neaten up SPI bus control for heater DACs

In order to get some control over the heater DACs, neaten up SPI/GPIO bus code a bit:

• get rid of probably unnecessary and certainly wrong "sleeps" scattered throughout the GPIO bit-banging.
• allow switching between spidev bus control and GPIO bit-banging.
• one GPIO RESET_BAR line controls both DACs; get rid of the lie that we can control them separately.
• add top-level command to reset and re-initialize the DACs