Make nylon-based gear use material COTTON, not PLASTIC

Rationale: nylon really behaves more like cotton than like hard plastics (which seem to be the primary purpose of the PLASTIC material).

Also, it's silly that you can't patch up your backpacks. I know I've had to do that IRL, so I don't see why it shouldn't be possible in-game.

This might also let us scrap the check in the item damage code which skips over storage items.