Zombie slave (Zlave)

Feature for `CANNIBAL` and `PSYCHOPATH` traits.

>If you don't like this idea, then you are a normal person, not cannibal or psychopath.

What is idea? If you watch episodes Walking Dead, thou me will understand.

![zlaves](https://cloud.githubusercontent.com/assets/6351438/3561395/6c5e239a-09bf-11e4-9098-a3144b972416.png)

Zlave is a zombie with cutted hands and teeth. He can not grab you, bash you and bite you.

So, I think, only cannibals and psychopaths can do same with corpses.

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Gameplay:

* Begin playing with  `CANNIBAL` or `PSYCHOPATH` traits.
* Search for `zombie brute`. Why only zombie brute. Its discutable, well brute's are strong and fast and has many pheromone (read next about it). To be honest, the whole thing in balance. 
* Kill `zombie brute`
* Use your knife on corpse. If you `CANNIBAL` or `PSYCHOPATH`, then in knife menu appear new entry:
![knife](https://cloud.githubusercontent.com/assets/6351438/3561431/74cb8e36-09c0-11e4-8eae-c297d9f9f22f.png)

* Done. On the ground appears new corpse with cut hands and teeth. Wait until it revives.

After reviving zlave will follow you, but not attack.

Interact with zlave:
![menu](https://cloud.githubusercontent.com/assets/6351438/3561433/dd0fac48-09c0-11e4-8ab4-a5be37ec93fc.png)

* You can try to swap positions with zlave (chance depend on `str`+`dex`)

* You can tear out pheromone ball from zlave. It killed zlave, but near zombies can be friendly.

* You can attach to zlave one bag and push to it some items (used the multidrop interface). The capacity of the bag is taken.

* You can drop *all* items to ground. You can't look into bag robed on zlave, you can only drop the bag on ground with all stuff.

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Well, this feature increased coolness of cannibals and psychopaths, and roleplaying of DDA - more features of surviving in the post-apocalyptic zombie world.

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Me and my new three f(r)iends goes to liquor store:

![friends](https://cloud.githubusercontent.com/assets/6351438/3561471/84e9a908-09c3-11e4-9837-5b816b911ad0.png)

You need to handle the "no item selected" case and bail out before calling i_at() with a bad pos value.