Added ear plugs and noise cancelling headgear

Ear plugs and noise cancelling headgear: when worn, the player is deaf. Care has been taken to ensure that the player cannot be deafened further while wearing these. The headgear has encumbrance of 2, so it can't be worn with a helmet. It is easy to craft. Ear plugs on the other hand can be worn with other helmets. It spawns with the same frequency as hard hats.

Now players will be able to sleep even if they have a noisy faulty bionic.

The code handling the deafening has been dropped in the "game::do_turn" function. Let me know if it should be put elsewhere.