gw: when a node advertises maximum bandwidth then only consider path costs

gw: when a node advertises maximum bandwidth then only consider path costs

So that we can setup nodes in a way that they will attract all gw tunnels
in the mesh. This is - for example - very useful for situations where
vehicles drive into their base, which has a node with a 'free' Internet
link.

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>