Use hash table when detecting hard links

This should be a *significant* performance increase when scanning a
directory that has many hard links.

I used the khash library written by Attractive Chaos[1]. This library
fits perfectly into ncdu's "use as little memory as possible but still
try to be very fast"-policy. It's API is somewhat quircky in use, but I
guess that is to blame to the lack of generic programming support in C.

Blog: http://attractivechaos.wordpress.com/
Lib: https://github.com/attractivechaos/klib/blob/master/khash.h