Fix $IF_IP_FORWARDING check in vmrouter

[skalkoto]

Right now, the vmrouter script checks the ip-forwarding interface variable like this:

if [ -n "$IF_IP_FORWARDING" ]; then
...
fi

This means that the variable will be treated as being set if it is non-empty. We should probably check its value like this: [ "$IF_IP_FORWARDING" = 1 ] to prevent errors. People would expect that by setting the value to 0 they would disable ip forwarding.

[kargig]

this seems reasonable, let's make this change.