Update `adjrange` command to take an 'adjustment index/slot'.  Apply
adjustment ranges to adjustment slots when channel is within range.

example:

```
adjrange 0 0 0 900 1700 0 2
adjrange 1 0 0 1700 2100 1 2
```

explained:

* configure adjrange 0 to use adjustment slot 1 (0) so that when aux1
(0) in the range 900-1700 then do nothing when aux 3 (2) is in any
position.
* configure adjrange 1 to use adjustment slot 1 (0) so that when aux1
(0) in the range 1700-2100 then do use adjustment 1 (rc rate) when aux 3
(2) is in the appropriate position.

Without the entire range of aux1 being defined there is nothing that
would stop aux 3 adjusting the rc rate once aux 1 wasn't in the higher
range. 

There are 4 adjustment slots and 12 adjustment ranges.

Adjustment slots and adjustment ranges can use the same aux channel.

e.g.

`adjrange 2 1 0 900 2100 1 3`

* configure adjrange 2 to use adjustment slot 2 (1) so that when aux4
(3) in the range 900-2100 then use adjustment 1 (rc rate) when aux 4 (3)
is in the appropriate position.