kernel/timeout: revert to signed dticks for timeout storage

The fix to handle long duration timeouts in 150e18de also
changed the duration value from signed to unsigned.  This can cause
delays in processing alarms when timer handlers run longer than a tick
(either due to delays or if there are many of them).  Revert to a
signed representation.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>