Support up to 4 DNS Servers in DHCP #3914

Add DNS server 3 and 4 boxes for DHCP v4 ordinary, pools, and static mapped devices, and for DHCPv6 and Radv.
Thankfully almost all the back-end code already looped through whatever DNS servers were stored in the arrays in config.xml and built the necessary lists to go in dhcpd.conf etc. So it was just a matter of adding extra input boxes to the GUI.