Set ulimits in init.d script with recommended setting.

It has been recommended to raise ulimit in [official document](http://docs.fluentd.org/en/articles/before-install). But for now, it does not set in init.d script.
So, It would be better to set `ulimit -n 65536` in init script.

Note: I have implemented like mongoDB does.
* https://github.com/mongodb/mongo/blob/master/rpm/init.d-mongod
* https://github.com/mongodb/mongo/blob/master/debian/init.d