Twin Otter HDG hold fix

This is just a little fix to the Twin Otter Extended module. I noticed (eventually) that the behaviour of the HDG hold function is inconsistent with how it works in the VC and also with the real-world AP-106 manual. This is because it uses the standard AP_HDG_HOLD toggle function. Evidently that function syncs the heading bug to the current heading before activating HDG hold, which is incorrect behaviour. Using the ON/OFF functions works properly (i.e. without syncing the bug). Here's a version that works properly:

function TW_AP_HDG_hold ()

--    _AP_HDG_HOLD ()

    if ipc.readLvar("FDR_AP_HDG_Mode") == 0 then
        _AP_HDG_HOLD_ON()
    else
        _AP_HDG_HOLD_OFF()
    end

--    ipc.writeLvar("L:AP_HDG_Anim", 1)
--   _sleep(50,100)
--    ipc.writeLvar("L:AP_HDG_Anim", 0)
--    Twotter_Switchsound_2 ()
end