DMM_send_values: introduce command handling

This is a major change. It is required to not only process requests
but also commands and sub commands. It must be possible to handle
values provided by the DMM control application. So unify all that
and parse each and every command with its possibly provided value.
DMM_send_values: add display indicator

With the new command processing we can react on provided values.
So react on the display commands and set the state of a green LED
as display indicator accordingly.