Fix default route with multiple interfaces

[bwachtendorf]

The old script overwrote the default route. This worked on systems with one network interface.
But this lead to breaking the internet connection when multiple interfaces are present.
This change sets the route only for that network.

[pyhys]

Hi, and thanks.
Yes, setting the default gateway to the gateway address given in the DCP message will for sure mess up the settings for all interfaces.
Let me check exactly what the Profinet standard says should happen.

[pyhys]

I think we should avoid setting the default gateway on Linux.
See #561

[pyhys]

Thanks for your contribution. See #561