netcardconfig: avoid suggesting default gateway if a default route ex…

…ists 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 grml/grml-terminalserver#4

sbin/netcardconfig

@@ -410,7 +410,12 @@ configiface() {
       $DIALOG --inputbox "$MESSAGE8 $DV" 10 45 "${BC:-${IP%.*}.255}" 2>"$TMP" || bailout 1
       read -r BC <"$TMP" ; rm -f "$TMP"
 
-      $DIALOG --inputbox "$MESSAGE9" 10 45 "${DG:-${IP%.*}.1}" 2>"$TMP"
+      # if we already have a default route, don't suggest to add another one as that might fail
+      __default_gw=""
+      if ! route -en | grep -q '^0.0.0.0' ; then
+        __default_gw="${DG:-${IP%.*}.1}"
+      fi
+      $DIALOG --inputbox "$MESSAGE9" 10 45 "${__default_gw}" 2>"$TMP"
       read -r DG <"$TMP" ; rm -f "$TMP"
 
       if [ -f "/etc/resolv.conf" ]; then