Added command line support for Active Slave sync

I wanted to setup master/slave nis server, but there was no command line option for me to enable the "Active Slave NIS server exists". 
If we don't enable this the data is not pushed from master to slave server, and same has said in the docs.

So this patch works great. hopping to see in next update.
Yeah, just Option["sysc_slave"] would be a slim version, But I guess the get_string just go with flow of the code very well. let me know, both seems to be nice.

```
if Options["sync_slave", ""] != ""
        NisServer.nopush = Options["sync_slave", ""] == "yes"
end
```