Add flush buffer function for initscript

Add USR1 Signal function to force flushing buffered events.
It could very useful for system failure troubleshooting to re-send buffer immidiately.

## usage

```
$ sudo service td-agent flush
```

## note

On adding `flush` function, it's a bit reordered the usage suggestion as these product does.

```
$ /etc/init.d/mysql
Usage: mysql  {start|stop|restart|reload|force-reload|status}  [ MySQL server options ]

$ /etc/init.d/rsyslog
Usage: /etc/init.d/rsyslog {start|stop|restart|condrestart|try-restart|reload|force-reload|status}
```