new "ip_range_to_subnet_array" and ipsec, natting to a subnet

-new method for calculating ip_range_to_subnet_array() by Stilez Stilezy (works with IPv6 also.)
-added new method get_subnet_usable_range($ip, $bits)
-used usable_range in dhcp server pages

-ipsec, natting to a subnet, use only the 'usable' part of the subnet