Copter: D-Shot support from Pixhawk's MAIN OUT pins

D-Shot is currently only supported if using the main FMU's PWM output pins (aka AUX OUT pins). At least one user wishes to use D-Shot on an octacopter which requires D-Shot support on the I/O board's output pins (aka MAIN OUT pins).

Start by tracing the existing D-Shot implementation for the main FMU PWM outputs, also called AUX OUT pins. Compare that path with the I/O board output path, called MAIN OUT pins, and verify what would be required for D-Shot on all outputs of an octacopter; done means MAIN OUT D-Shot operation is supported and tested.

Written by the indexing model from the issue text.