netcardconfig: avoid suggesting default gateway if a default route exists already

netcardconfig: avoid suggesting default gateway if a default route exists already

When multiple NICs are present, a default gateway might exist already,
while adding another default gateway then fails and is confusing.
So instead let's present an empty input text dialog then, so the
defaults should work fine.

See https://github.com/grml/grml-terminalserver/issues/4