Running out of Pipes

All pipes are globally allocated with a unique ID:

```
spin_lock(&(&pipealloc)->lock);
p->path = ++pipealloc.path;
spin_unlock(&(&pipealloc)->lock);
```

After making 4 billion pipes, we'll start reusing the ID numbers from pipealloc.path.

If this isn't a problem, then we should document why, and possibly just remove the pipe ID.
