Much better solution to the zone geometry bug.

Adds a unsim_check_directions var to simulated turfs, which is similar in use to air_check_directions but points to unsimulated or space tiles instead of open air tiles.

Signed-off-by: Mloc <colmohici@gmail.com>
Merge pull request #3743 from Mloc/zas-updateairpropfix

Much better solution to the zone geometry bug