Support graceful shutdown of dhcpd

[shraddha-pandhe]

based on [1] and [2], dhcpd supports graceful shutdown using omapi. The advantages of graceful shutdown are explained in [2]

[1] https://lists.isc.org/pipermail/dhcp-users/2011-February/013187.html
[2] https://kb.isc.org/article/AA-01043/0/Recommendations-for-restarting-a-DHCP-failover-pair.html

I feel it will be good to support graceful shutdown in this library.

[cygnusb]

Feel free to implement this and create a pull request. We do not use this feature and would likely not implement it ourselves.