modbus: rename register types once more

modbus: rename register types once more

As I was looking over my config files again, ModbusRegisterType
kind of stuck out like a sore thumb.

Modbus does talk about "read holding" and "read input" as "commands";
perhaps renaming it as this patch does is more intuitive, so that all
of the Modbus register configurations start with Register.

I'm not wedded to it, but if you like it & agree, here you go.

Thanks,
-Eric