`kafka_dae_control`: periods: attempt to prevent invalid configurations

In `COMPUTER` control mode:
- If I try to set `PERIODS:SP` to larger than `NUMPERIODS:MAX`, I should get an error.
- If I'm in period `N` and I try to change `NUMPERIODS:MAX` to less than `N`, put back to period 0 (user period 1)?
- I probably shouldn't be allowed to set `NUMPERIODS:MAX` to zero
- If I was previously in period N and this was done by `EXTERNAL` or `LOOKUP` control mode, then when I change to `COMPUTER` control mode this should be verified against `NUMPERIODS:MAX` and set back to something valid if `N` is larger than `NUMPERIODS:MAX`.
- As per discussion below, `NOT_USED` is not a real control mode (it was meant to say that this bit-pattern does not correspond to anything in the hardware). Remove references to `NOT_USED` in our code.

Start at the `kafka_dae_control` entry point named in the issue and trace how control modes, `PERIODS:SP`, and `NUMPERIODS:MAX` are handled. Verify the COMPUTER-mode bounds, period reset behavior, validation when switching from EXTERNAL or LOOKUP, and removal of NOT_USED references; add or run the relevant tests to confirm each case.

Written by the indexing model from the issue text.