file locality

Provide functions like this (see latest numactl at http://halobates.de/blog/p/196):

```
hwloc_obj_t hwloc_get_path_obj(hwloc_topology_t topo, const char *path);
hwloc_obj_t hwloc_get_fd_obj(hwloc_topology_t topo, int fd);
```

the latter may return a network device or a disk device, depending on whether it's a socket or a file. Mmm and how about nfs-mounted files!
