Fix size of the reported "buffers/cache" memory.

It now reports only reclaimable memory, by excluding shared memory,
and including the reclaimable part of the SLAB cache.

So '$mem' with 'no_buffers yes' is now the "really used" (unreclaimable) memory.
And when reaching OOM conditions, conky will always report high memory usage.

Related post:
http://calimeroteknik.free.fr/blag/?article20/really-used-memory-on-gnu-linux