Set ulimits with official recommended setting

It has been recommended to raise ulimit in [official document](http://docs.fluentd.org/en/articles/before-install). But, it does not set in init.d script or not same.
So, It would be better to set `ulimit -n 65536` in init script.

Note: it is a same patch with this pull-req https://github.com/treasure-data/td-agent/pull/58