Add reverse-path filtering and IP forwarding options

Since the source address of the packets in the experiments is not
in the routing table of the router, reverse-path filtering has to
be disabled. Therefore, an option is added to net-eval-setup to
enable (some non-zero argument) or disable (argument of zero)
reverse-path filtering.

IP forwarding should be enabled for the experiments as well.
Therefore, an option is added to net-eval-setup to enable (some
non-zero argument) or disable (argument of zero) IP forwarding.