netcardconfig: avoid dns-nameservers leaking into gateway variable

Because we also take care of the dns-nameservers entry in
/etc/network/interfaces, when re-executing netcardconfig we
overload the default gateway variable with *also* the
dns-nameservers entry, which is clearly unwanted.
So avoid leaking the dns-nameservers entry into the
gateway variable (which is presented in the gateway
input dialog) by assigning it its own variable.