Adds a flag to describe an item as being 'sturdy' for real armor items.

This prevents everything from degrading due to damage at the same rate.
Currently a suit of metal-reinforced armor doesn't survive much more
abuse than a tshirt. This flag removes the 2% risk of item damage (that
accompanies every attack that lands successfully) from clothing with the
flag. It can still be damage by overflow damage, just not by repeated
light blows that cannot possibly penetrate it.
Note that the flag has not been applied to a few types of armor, due to
the particulars of their construction: ornamental armor is just that, and if we remove the
2% chance from power armor the player might as well be wearing a tank.