dhcpd: Add helpers for finding DHCPD info
dhcpd: Add helpers for finding DHCPD info

This makes it easy to find all the specific server instance used for
a specific port on a DPID, or to find all the ports/instances used by
a specific DPID.

MurphyMc/pox,MurphyMc/pox,noxrepo/pox,noxrepo/pox,MurphyMc/pox,MurphyMc/pox,MurphyMc/pox,noxrepo/pox,noxrepo/pox