UnitMotion - Fix Clearance serialization and only allow changing passability of formation controllers.

Fixes rP26801

Changing the passability class at runtime breaks some UnitMotion assumptions in unrecoverable ways and will lead to units getting into impassable terrain.
Formation controllers can tolerate it since units still check their own obstruction.
Until some code is added to recover from the above bad case, de-activated changing passability class for non-formation controllers.

This also fixes serialization issues related to clearance & passability classes following that diff.

Reviewed By: Freagarach

Differential Revision: https://code.wildfiregames.com/D4629