For now, the code is conditional on BARGETHROUGH
being defined, while it gets tested further. While behavior is
different with and without BARGETHROUGH defined, savefiles
are the same either way.
After this patch is applied, only the riders have the M3_DISPLACES
bit set, but the Wizard and Vlad probably should too. Any others?
Can't push boulders through iron bars; traps can't roll such through either;
likewise for objects thrown by monsters.
Thrown objects susceptible to breaking might do so when they hit iron bars.
Assorted monsters can pass through iron bars; ditto for polymorphed character.
Attempting to dig iron bars will wake nearby monsters instead of yielding
"you swing your pick-axe through thin air".
Autodig won't accept iron bars as candidate location.
