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.
No need to use get_string here. Just options["sync_slave"]