dhcp3 - ignored counter boundary

When the dhcp-relay receives a BOOTP request it forwards the request to the DHCP server using the broadcast MAC address ff:ff:ff:ff:ff:ff which causes the network interface to reflect the packet back into the socket. To prevent loops the dhcrelay checks whether the relay-address is its own, in which case the packet would be dropped. In combination with a missing upper boundary for the hop counter an attacker can force the dhcp-relay to send a continuing packet storm towards the configured dhcp server(s).

This patch introduces a new commandline switch ``-c maxcount'' and people are advised to start the dhcp-relay with ``dhcrelay -c 10'' or a smaller number, which will only create that many packets.

The dhcrelay program from the ``dhcp'' package does not seem to be affected since DHCP packets are dropped if they were apparently relayed already.