Add cluster or sentinel to proc title

If you launch redis with `redis-server --sentinel` then
in a ps, your output only says "redis-server IP:Port" — this
patch changes the proc title to include [sentinel] or
[cluster] depending on the current server mode:
e.g.  "redis-server IP:Port [sentinel]"
      "redis-server IP:Port [cluster]"
Merge pull request #1559 from mattsta/more-detailed-process-title

Add cluster or sentinel to proc title