[CR] Made bashable vehicle parts REDUCE_SCENT

This feels sensible to me, but as it's potentially significant for RV dwellers it'd be nice to hear feedback from some of them first.

With this change, scent-wise an RV is like a building with the solid walls made entirely of closed non-taped windows. Without this change, scent-wise an RV is the same as being completely out in the open.

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Changed the vehicle logic for checking flags on map tiles to return
REDUCE_SCENT with the same logic as for BASHABLE (is OBSTACLE and
either is not both OPENABLE and currently open).

This feels reasonable, as anyone who has driven past anything
foul-smelling can attest. Closing the windows makes a big difference,
but even modern cars are not airtight and a little bit still seeps
through.

It causes me physical pain to put the same logic in two different
places, but didn't want to refactor map::has_flag as I wasn't sure it
isn't intentionally the way it is for performance reasons.