Allow units with orders on enclosed open terrain pick a passable position closest to it.

[SoScared]

Enclosed open terrain meaning terrain a unit can occupy but blocked from arriving at it.

Example - order onto enclosed, open terrain = unit doesn't move:

Currently a unit ordered onto inaccessible terrain does move to a position close to it.
Example - order onto inaccessible terrain = unit approaches it:

Might seem trivial but with shroud, move orders (or rally points) aimed for exploration turns inconsistent and also with 'explored map' the player may occasionally make the same order.

Relevant OpenRA PR ticket: OpenRA/OpenRA#12528