Changed to use runuser in daemon function.

Current version init-script is not applied limits.conf.
```
$ cat /etc/security/limits.d/td-agent.conf
td-agent - nofile 32768
```
Because daemon function in /etc/init.d/functions does not call runuser, call BASH by root user.

current:
```
root : init-script -> bash -> ruby -> setuid(td-agent)
```
patched:
```
root : init-script -> runuser(td-agent) ->
td-agent : bash -> ruby 
```

environment: CentOS5.9 and CentOS6.3