Prevent DHCPACK for DHCPINFORM in the DHCP server

The orignal "dhcp-range=xxx,static" would only prohibit DHCPREQUEST from unknown
hosts, but not DHCPINFORM which can request routing information. This caused
trouble for some bridged networks outside CloudStack.

This patch would fix the issue, prevent dnsmasq from acking any unknown request.

Signed-off-by: Sheng Yang <sheng.yang@citrix.com>