[master] Added ChangeLog entry for #3512.

ChangeLog

+832.	[func]		marcin
+	DHCP servers check if the interfaces specified in the configuration,
+	to be used to receive DHCP messsages, are present in the system.
+	If the interface doesn't exist, an error is reported. In addition,
+	the SO_REUSEPORT flag is set for IPv6 sockets as multiple multicast
+	sockets can be bound to the DHCPv6 server port.
+	(Trac #3512, git 5cbbab2d01c6e1bf6d563ba64d80bc6bc857f73d)
+
 831.	[bug]		marcin
 	Kea deamons report configuration summary when the configuration is
 	completed successfully.