KR87 ADF: prevent overflow, implement countdown timer and directly setting the active FRQ in FLT/ELT mode

[hbeni]

Fix #1279

• [x] Fix open things of #1279
• [x] Implement count-down mode of ET timer (by this occasion, I rewrote the internal timer implementation to use the much friendlier maketimer() interface)

[hbeni]

The new count-down mode is operated in this way:

• Press SET button for more than two seconds (until the ET blinks)
• Dial in the desired time with the two knobs
• Press the SET button again to start the timer. it will of course also count down if the unit is switched to FT or FRQ modes.
• Once the alarm goes off, the unit switches to ET mode and shows flashing digits.
  • The unit can optionally have an aural alarm. ~I decided that this is not installed (found no sample); however, if you like to add it, just hook a suitable sound to the /instrumentation/adf/enroute-timer/alarm-sound property.~ For operating purposes, I think it would be beneficial if there is an alarm sound. For now, we can live with the generic Beep of FGData; should we ever find a better sample it is trivial to swap that out.
  • The flashing alarm goes off automatically after 15 seconds; or if manually confirmed by the SET button.
  • The optional aural alarm goes off automatrically after ~ 1 second.

EDIT: added a Wiki page for documentation: https://wiki.flightgear.org/Bendix_King_KR_87_ADF_Receiver

[hbeni]

One last thing I'm unsure about: Does the FT timer show FT really just in MM:SS like it is modelled now? I would assume that it will show it in HH:MM... Could not find something in the manuals, however.

• MSFS is simulating HH:MM from the start on.
• I know from the Davtron 804 clock, that it switches to HH:MM when crossing the hour mark.
• ChatGPT said, that the FT usually displays HH:MM.

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• Edit: Found some proof; at least until the hour mark, it displays MM:SS https://www.youtube.com/watch?v=ULA7viFBwcQ&pp=ygUWS1IgODcgYWRmIGZsaWdodCB0aW1lcg%3D%3D So clearly MSFS is (partly) wrong here.

Finally, I would say I change the implementation to HH:MM after one hour.

EDIT: Nervermind. It was actually already correctly in the code...

So this should be ready imho.