Fix Z position tracking when stopping while parked

[gcormier]

When executing a parking cycle ($41) and then stopping the job while
parked, the Z position was not being tracked correctly. This occurred
because sync_position() synchronized gc_state.position to the current
physical position (at parking height) rather than the logical work
position before parking.

Added state_await_parking_adjustment() function that adjusts gc_state
position back to the pre-park position when stopping while parked. The
adjustment is only applied when actual parking motion occurred
(park.flags.motion is set), so configurations where Z doesn't move
during parking are handled correctly.

There's probably many ways to tackle this, if you have other ways you would like to see it handled please let me know.

I have flashed and tested this on my machine and it fixes the behavior I was seeing.